To equip a new ball you just need to select which one you want from the BALLS menu of the equipment screen. Bring your GI-SD balls to the front then click and drag the ball to the ball spot in the top right hand corner or your bag. You don't even have to take the old ball out, you can just drag the new ball on top of it to replace it.
Same thing for any clubs as well. Just drag the new club into the proper slot and release the mouse button to replace the old clubs.
You can also watch a video
tutorial of how to equip your bag. Just click the link.