From the Bill Pay main screen, click on Options below the name of the payee you would like to rename.  In the picture example below, to change "Jays Plumbing" you would click on Options directly below Jays Plumbing.

On the Options screen, click on the Account Information tab, then edit the Nickname on the right.  If you want the display name to be the actual name of the payee, erase the nickname field and leave it blank.

Once you have changed or erased the Nickname, click Save in the lower-right corner.