go to https://www.oneplus.com/customer/info and log in with your account, username can only be changed once.
nvm that special builds is not required for the 5t, but you should be able to revert back to oos 4.7.x using the unbrick tool if you can't do it with...
first off, you should use oreo compatible twrp
use fastboot to...
boot to twrp
adb push update.zip /sideload
install the zip then format internal storage
use a browser if you want to edit a thread, op community app is still buggy and shouldn't be used in the first place.