Members upgrading from one level to another

If someone signs up for the Level 1 Basic or Level 2 Super and wishes to upgrade to the next level... how does that work/how do they do that? (as they'll have already paid a fee for the first level, so should only pay the next tier fee minus the first level fee or the top tier fee minus the first or second level fee... if that makes sense?) Do I need to set a specific page for 'upgrading'?

Thanks
I look forward to hearing from you.

Kind regards
Cheryl Rickman