When you purchase new equipment and the transaction is complete, there is a place to click that says " Equip your bag" . either that or before you tee off for a round, there is a place that says " view/edit my clubs. That will take you to the same screen. This will show all the clubs and balls that you own. You can either click on the clubs that you want to change at the top of the screen or click at the bottom of the screen where the different clubs are listed. This will show you all of the clubs that you currently own. For instance if you onw 3 drivers they will appear in the middle of the screen and one of them will say "equipped", meaning that this is the one you are carrying in your bag. To change clubs, click the green arrow to either the left or right until the club that you want to use is in the middle. then drag the club up to the top of the screen and put it where it belongs in your bag. Then hit save changes and you should be good to go.
Keep it on the short grass.