Dave, you are correct that when you have a club in your bag you can't just put a new one in. That is why you go to the top right of the start page and click on your id picture.
Now you will see toggles at the bottom of the new page (Equipment & Stats page) with one for editing equipment. Once you click on this toggle you will get a page with equipment choices with your drivers highlighted. The top line will be the one in your "bag" and and to the right a picture of that driver with a toggle saying "Equipped". The others that you own will be in the line below. If you have purchased a new driver it will show here. (WGT also makes it possible to make a purchase from this page so be wary.)
To change, click on club you want to put in your "bag". It will now be shown to the right with a toggle saying "Equip". Hit that toggle and you have just changed clubs. Leave the page by going to the top left and hitting the left arrow point.
Side bar: If you want to change your id pic go to the stats & equipment page and click on your id pic which is at the top left just over the column of equipment.