A branch is just a pointer to a commit, so you can freely move it around. To make it so that the branch is referring to the commit
aabbcc, issue the command
git reset --hard aabbcc
Please note that this will overwrite your branch's current commit, and as so, its entire history. You might loose some work by issuing this command. If that's the case, you can use the reflog to recover the lost commits. It can be advised to perform this command on a new branch instead of your current one.
However, this command can be particularly useful when rebasing or doing such other large history modifications.