git - How to reset the origin/master ref locally -

i have following scenario:

---

1. i have 3 commits in local repo, this:

   initial -- -- b

   where both master , origin/master point @ b.

2. after that, fetch origin, , repo looks this:

   initial -- -- b -- c -- d

   where master points @ b , origin/master point @ d.

3. after that, merge , master points @ d.

---

i can reset master branch doing git reset --hard b.

what reset origin/master ref commit b. and, not mean want reset remote repository state - want locally only. no pushing. want remote remain unchanged. want change knowledge of local repo regard state of remote repo.

how can that?

i can change files in .git directory, files? or there maybe git command it?

why need that? because want able replay scenario, test git-based project management i'm working on. replay same situation, in order see how behaves if change conditions slightly.

the safest way git update-ref refs/remotes/origin/master <sha>. cause change recorded in reflog, should ever need refer it. though codewizard said, next fetch update ref anyway, there not risk in changing directly either.