Once a merge is completed, you can't unmerge, it's not actually possible. All you can do is recreate the other person, add/remove relationships to the two profiles and edit the bios to get back to where you were. If you have not rolled back or edited the bio since the merge, there will still be two distinct bios under the one person. If you have rolled back, then info could be lost.
If however the merge is incomplete and is still only a proposed merge, you can set it as a rejected match so it doesn't happen again by someone else. Don't remove the rejected match, it's there to notify others.