option for members to upgrade (multiple memberships)

Hi.
I am unsure how to give usesrs with Multiple Memberships option to upgrade their plan to a different track. Anyone done this before? This is what the documentation has – https://www.armemberplugin.com/documents/how-to-addchange-membership-plan/… However its unclear what to do in the case of multiple memberships.
I would like there to be a page where memebrs can select a different plan within their membership and automatically have their access changed and payment changed.
Thank you