I wrote a guide on how to update .patchsets. You can find the post about it here.
I wrote it to have a step-by-step guide based on a real example. Not the most clear/readable thing on the planet (mostly because it assumes certain familiarity with HaikuPorter, git, etc), but I follow that procedure all the time now.
In any case… you first need to understand that Haikuporter downloads the a source tarball, extracts it into a “work-dir”, then it initializes a git repo there, adding all the original files from the tarball. It tags it “ORIGIN”, so you can work there as you please… edit, commit, etc, and either revert back to “ORIGIN” if something goes wrong, or ask HaikuPorter to extract (haikuporter wine -e) a .patchset file containing all the commits you made.
if a .patchset already exists, but fails to apply (as in git am fails)… you can either fix things manually, of (try to) follow the guide I’ve mentioned above.
Good luck!