You said you already installed Ubuntu Touch.
I'm presuming that you have unlocked your bootloader before doing so and that you might have used the terminal and might be running Ubuntu.
If this is correct it should be no problem for you to:
( I have tried reaching out to TingyiChen via XDA private message 8 days ago but got no response since then. )
As TingyiChen currently seems to be our official device maintainer but lately we haven't heard any statements regarding bugs, issues or...