I have an FPC fork on GitLab to create pull requests. How do I update that to the current FPC version? It isn't a paid account, so mirroring doesn't work.
And you have a clone of that fork on your local PC?
Add the original FPC gitlab url as a 2nd remote to your local clone.
git remote add fpc http://......
(or use your git gui...)
Then you can pull or "git remote update" from each remote.
You can then push to your fork.
For this you should have a local branch that "tracks" origin/main (assuming your fork remote is called "origin" / as is default / "git remote -v")
The easiest is probably if that branch is NOT your work branch.
That is if you committed your changes to "main" (local branch main) then you may use another name.
git switch -c fork-main --track origin/main
git merge --ff-only fpc/main
git push
If the merge doesn't work: git stash push ; git switch -f -C fork-main fpc/main ; git stash pop
Note, this form of git switch will not ask questions, it clears changes in your work tree, and move the branch, which in other scenarios can mean commits that are only on that branch are "kinda dropped".