This step is only applicable if you would like to give members the ability to automatically upgrade from one member type to another through the Member's Only Area. To setup upgrade fees, go to the Setup->Member Settings menu, and at the bottom of the page you will see:

Complete the form shown above for all different upgrade fess you require. Through the Member Area, members will be able to automatically upgrade their membership, and will be shown all options you create in this section. Members will be able to choose the desired option, immediately pay the appropriate fee, and will be automatically upgraded to the appropriate member type.