The way members upgrade their membership depends on if you are using direct payment or not. Direct payment is a Pro System setting for the commission tracking tool where members are paid direcly. By default your system is set to "One Payment to Admin".
One Payment To Admin
By default, on the order page (your-domain.com/order) payment buttons appear where a member can upgrade their membership. You can also use the ~~ codes provided for pasting the buttons on web pages (capture/landing/etc).
To make a button for members to upgrade their membership Go to: your-domain.com/admin/managebilling Click edit payment option and copy the URL under URL's to Promote -> Upgrade URL