Newbie help: Marshmallow update - AT&T HTC One (M8)

Please forgive newbie ignorance. I rooted my AT&T HTC One M8 about a year ago with very little knowledge and the assistance of a "dummies" guide so I could run Titanium backup. In that time with no OTA updates, my phone's become glitchy and I want to get it to a new Marshmallow base. I don't really care if it's a custom ROM or the recent stock ATT update, just want to get everything up to date and stable.
I have TWRP and SuperSU installed. My CID is stock. I am S-ON and the phone is unlocked.
What is the easiest way for me to get my phone up to date?

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Need more info. Do fastboot getvar all, and post the output (delete IMEI and serial number before posting).
Also, do you actually use AT&T as your cellular provider, or no?
Without knowing more, I'd say that flashing a custom ROM is probably the easiest way to get to MM. But I can't say for sure whether you are configured with the right firmware to run the current ROMs or not, without the info requested above.

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(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: [email protected]
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 4.28.502.1
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: xxxxxxxxxxxx
(bootloader) imei: xxxxxxxxxxxxxxx
(bootloader) imei2: Not Support
(bootloader) meid: 00000000000000
(bootloader) product: m8_ul_ca
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B12000
(bootloader) cidnum: CWS__001
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 7f8e91a3
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
ATT is my current provider and the ROM installed is the stock ATT Lollipop

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breid77 said:
(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: [email protected]
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 4.28.502.1
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: xxxxxxxxxxxx
(bootloader) imei: xxxxxxxxxxxxxxx
(bootloader) imei2: Not Support
(bootloader) meid: 00000000000000
(bootloader) product: m8_ul_ca
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B12000
(bootloader) cidnum: CWS__001
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 7f8e91a3
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
ATT is my current provider and the ROM installed is the stock ATT Lollipop
Click to expand...
Click to collapse
1 .If you want to bring the phone back to stock you will have to relock bootloader and run this ruu http://androidruu.com/getdownload.p...40.45.C33065.00_F_release_426617_signed_2.exe then try ota updates that should bring you to stock att marshmallow but keep in mind it's not that easy to root when you are on marshamallow but it's not terrible.
2. If you want custom rooted marshamallow roms just head to HTC One m8 Android development , there are plenty of roms there. Good luck.
Sent from my HTC One_M8 using Tapatalk

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Will I have to gain S-OFF to relock my bootloader?

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No. Just google how to do it
Sent from my HTC One_M8 using Tapatalk

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grigoriisi said:
but keep in mind it's not that easy to root when you are on marshamallow but it's not terrible.
Click to expand...
Click to collapse
Rooting MM is easy, and same basic process as LP: unlock bootloader again, flash TWRP, (highly recommended to make a TWRP backup), then flash SuperSU. Only difference, is that you need to make sure to use proper version TWRP (3.0.x) and SuperSU 2.65.
In particular, using an old (not updated for MM) version of SuperSU will cause a bootloop or no boot. But its basic good practice to always check and use the proper recovery version and SuperSU version any time you root.
---------- Post added at 09:11 AM ---------- Previous post was at 09:10 AM ----------
breid77 said:
Will I have to gain S-OFF to relock my bootloader?
Click to expand...
Click to collapse
No, s-off is only needed to make it say LOCKED for folks that want warranty service.
To make it say RELOCKED (which is acceptable for RUU) is a single fastboot command (and s-on is fine):
fastboot oem lock
---------- Post added at 09:16 AM ---------- Previous post was at 09:11 AM ----------
The fact you mention relocking the bootloader, means its currently unlocked. And to me implies you currently have TWRP installed (if so, what version?).
If that is the case, and a relatively updated TWRP version, it would be a simple matter to put a MM custom ROM on the phone or SD card, and flash it in TWRP.
MM ROMs will run fine on the current setup. Its really your choice whether you want to flash a MM custom ROM, or return to stock by RUU and OTA to MM. But if you want less steps, custom ROM is probably the way to go.

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My TWRP version is 2.8.6.1.
I'd be fine with a Custom Rom. I have no idea which ones are stable and support my hardware, though.

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All marshamallow roms from one m8 Android development will work fine on your current device . Just browse and read.
Sent from my HTC One_M8 using Tapatalk

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breid77 said:
My TWRP version is 2.8.6.1.
I'd be fine with a Custom Rom. I have no idea which ones are stable and support my hardware, though.
Click to expand...
Click to collapse
TWRP version is probably fine, but best to update to the current version, 3.0 (flash TWRP by fastboot). Required files and instructions here: https://twrp.me/devices/htconem8gsm.html
Any of the ROMs intended for the AT&T M8 or the "international" version will work. Any of the Sense ROMs should be stable. "Stable" being a subjective term, but they should all be fine as daily drivers. Its more of matter of what features you want, and that is the fun bit about custom ROMs, so its really your choice!