This is how I did a similar upgrade a few months ago:
You can find 3.1.1-A15 here (in the url line in the fifth big block):
https://ota.ecloud.global/api/v1/FP3/community/
In my case, the starting point was official and a locked bootloader. If you are on community and have an unlocked bootloader, this should not make a difference here (it should actually keep things simpler).