On the Players tab, edit the incorrect player. On the Edit Game Player dialog that opens, near the top in the "Info" section, it will say "This player is in your player database.", and there should be two buttons to the right on the same line: "Change" and "Unmerge". Press the "Change" button. The "Change Player" dialog will open. Select the correct player. Press the OK button, then press the OK button on the "Edit Game Player" dialog.
That's it. No need to undo anything, buy-in anyone else, or anything like that. Just swap the wrong player for the right player.