[Q] Android won't boot after modifying system file - Android Q&A, Help & Troubleshooting

This is (nearly) my first post, so hi all !
Before I start with anything else: I searched a lot and found a lot of things, but I'm so newbie that I'm afraid to do anything because I don't quite understand what I'm about to do.
Oh, and I don't speak english natively so forgive me if I make any horrific mistakes :angel:
With all that said, let's begin with my problem...
I have a HUAWEI Y300-0100, and it comes with a funny feature: every time it can't connect to WIFI a popup message appears saying that if you switch to data plan charges may apply.
I am constantly switching between WIFI/no-WIFI zones so that message really bothered me, and I searched a way to disable it.
Presumably, the only way to do that is by modifying a system file called "build.prop". I already had my phone rooted so doing so wasn't very hard.
I used Jota+ to change a setting from "true" to "false" (ro.config.hw******), and saved the file in the root of the SD card. Then, with ES File Explorer, I renamed the "build.prop" in /system to "build.prop.bak" and copied there the "build.prop" I previously saved in the SD root.
All seemed to be fine, except that after rebooting my phone it could not get past the HUAWEI logo.
Android recovery mode seems to work, but I don't know how to use a lot of its options nor I want to do a Factory Reset if there's a better alternative.
Is there a way to modify system files before booting Android? I really would like to try to delete the "build.prop" file and rename the "build.prop.bak" back to "build.prop".
Does any of that make any sense? What do you recommend me to do?
Thank you very much for your help!

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From within recovery open adb
adb pull /system/build.prop /some/location/on/PC
Now edit build.prop back to how it was
adb push build.prop /system
adb reboot
Sent from my Nexus 4 using Tapatalk 2

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demkantor said:
From within recovery open adb
adb pull /system/build.prop /some/location/on/PC
Now edit build.prop back to how it was
adb push build.prop /system
adb reboot
Sent from my Nexus 4 using Tapatalk 2
Click to expand...
Click to collapse
Thank you for your response!
I managed to restore the original build.prop, but the phone still doesn't boot.
..Maybe that wasn't the problem. I guess I'll factory reset...
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After that it STILL doesn't boot. I have no idea what happened, I did nothing strange apart from editing that file :crying:

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If you changed something in /system factory reset won't work
Try either fixing permissions in recovery or
adb chmod 644 /system/build.prop
Sent from my Nexus 4 using Tapatalk 2

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demkantor said:
If you changed something in /system factory reset won't work
Try either fixing permissions in recovery or
adb chmod 644 /system/build.prop
Sent from my Nexus 4 using Tapatalk 2
Click to expand...
Click to collapse
Nothing fixes it... I give up.

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Next time be sure to make a backup before you modify
For now wipe clean and install a new ROM or return to stock
Sent from my Nexus 4 using Tapatalk 2

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Install a new rom
Sent from my GT-I9100G using xda app-developers app