In Playbook Wizard the center is the offensive player who starts the play with the ball.  That player is indicated by a small football icon at the bottom right of their player shape:

If you right-click on the center then hover over the Player sub-menu you’ll see that the center has “Starts Play with Ball” checked:

To change which player is the center, right-click on the player, hover over the Player sub-menu, then click “Starts Play with Ball”.  That player will now be the center.