This article explains how to insert or change pitchers while tracking pitch counts during a live game.
Whenever you would like to change pitchers while tracking pitches within ChangeUp simply click on the BULLPEN button in the bottom left corner of the pitch count screen. This will display a list of all of the players on team, their availability status indicator, and when they last pitched. To insert a pitcher into a game, click the select box to the right of the pitcher's name and then click INSERT PITCHER.
If you want to insert a pitcher that is not already part of the team roster within ChangeUp you can do so via the EDIT ROSTER button. See How do I edit my team roster? for more information.