1. While using the Draft Assistant or Draft Simulator, move your cursor over to the player you want to move on the "Your Team" pane on the right.
2. Click on the drop-down arrow that appears.
3. Click on the position you want to move that player to.
Once this is done, the player will then be sent to your desired position.
You can only do this for your team. The opponents in the Draft Simulator will automatically move players around to make sure that all starting spots are filled.