I have the following situation in my project:
I would like to get rid of the merge commit (c6ac755) and rebase master on b35dac5 instead. To this end, I would like to reset origin/master to b35dac5, rebase master on origin/master and then synchronize with GitHub. How can I reset origin/master without changing master and pushing it to origin?
It looks like you merged origin/master locally (probably with git pull), pushed the merge and then reset the local master branch with something like git reset --hard HEAD~1.
In this state, you can use push to set the remote branch to the desired commit:
git push --force origin b35dac5:master
After this, you can rebase master on origin/master:
git checkout master
git rebase origin/master