I want to change my subscription monthly price

No problem! Just view "All Model Options" on the left hand menu. Then select the drop down for "Pricing." 

When changing pricing, you can update it for new subscribers only or for all past subscribers. If you choose the latter, subscriptions ending within 7 days keep the OLD price; those ending after 7 days pay the NEW price. 
 

To adjust this option, set "When a fan rebills, they are charged:" to "The Price When They Signed Up" or "My Current Price" as your preference dictates.