On the main menu of the game window the 2nd drop down from the right is my settings. Under that click on my equipment. That's your bag. Click on your starter 3 wood under optional clubs. Change the club to the one you just purchased. Drag that club to where your 3 Wood is now under optional clubs. Click on save changes.
If none of this makes any sense then there is a tutorial in the FAQs section on the right hand side of tis page. It's the how to equip your bag one.
Good luck.