If you want to merge up to a specific commit, fetch all remote changes and specify this commit's id:
git fetch origin
git merge <sha1 of commit you want to merge>
You can specify the --ff-only
option and have Git fail when it cannot fast-forward (and would create a new merge commit).
If you only want to put a single commit on top of your current branch's tip, use cherry-pick (but this is usually not recommended):
git cherry-pick <sha1 of commit you want to merge>