Previous answers assume that the branch to be reset is the current branch (checked out). In comments, OP hap497 clarified that the branch is indeed checked out, but this is not explicitly required by the original question. Since there is at least one "duplicate" question, Reset branch completely to repository state, which does not assume that the branch is checked out, here's an alternative:
If branch "mybranch" is not currently checked out, to reset it to remote branch "myremote/mybranch"'s head, you can use this low-level command:
git update-ref refs/heads/mybranch myremote/mybranch
This method leaves the checked out branch as it is, and the working tree untouched. It simply moves mybranch's head to another commit, whatever is given as the second argument. This is especially helpful if multiple branches need to be updated to new remote heads.
Use caution when doing this, though, and use gitk
or a similar tool to double check source and destination. If you accidentally do this on the current branch (and git will not keep you from this), you may become confused, because the new branch content does not match the working tree, which did not change (to fix, update the branch again, to where it was before).