The process of cleaning up your player database can be done by using the Move Player Statistics option on the Tools menu. Please check the Help menu for info on this Tool.

Also, below is some additional info on using the Tools->Move Player Statistics option...

Lets say you have a player name misspelled and then the correct entry of the players name, I will use Brian as the example.

On the FROM side, select the team and then the incorrectly spelled Brian.
On the TO side, select that same team and then the correctly spelled Brian.
Click Move to move all the stats.

Close out of move player stats, go into the Roster for that team and delete the misspelled Brian from the team (right side Delete button). Then find the misspelled Brian on the left side all player list and delete him from there. If you get a message about deleting stats from a team when attempting this delete, make note of the team name listed. The incorrectly spelled Brian was used on another team so you will need to move the stats on that team as well, click X to close out of the message and use the Move Player option for Brian on that team(you may need to add the correctly spelled Brian to that teams roster prior to using the move command).