To simply remove all previous rerere resolutions, run rm -rf .git/rr-cache to remove the cache.
For a specific merge, you can tell rerere to forget the recorded resolution by re-executing the merge and allowing rerere to apply its recorded resolution in the work tree.
You can check out a' and then do a git merge b to get back into that situation (you’ll probably be in a checkout of a detached head since you specified the commit hash of a', so be aware that you’re not on a branch).
Then use git rerere forget FILE-WITH-BAD-MERGE where to specify the file whose recorded conflict resolution should be forgetten.
forget <pathspec>
Reset the conflict resolutions which rerere has recorded for the current conflict in .
(From the Git documentation for git-rerere.)