Replacing a player
For future reference or if this was not possible because you only found the error after some rounds had been paired, the most useful way to accomplish this edit on WER is to export the tournament to XML, open it in your favourite word processing program and use a find/replace to amend the offending entries (but probably without using a Replace All especially if it's a common name!)
I'll mention also that while it may be good customer service in your local store to keep track of players' DCI numbers so they don't have to remember or quote them to register, it's less good for the players who go to play at another store or a GP without their DCI numbers, not to mention problematic for the scorekeepers there. Please get your players to store their DCI numbers in their cellphones, as a contact called DCI Number!