You may be unable to edit memberships even when logged in as an admin. The fix depends on the exact issue, as explained below.
No Products Menu Option
Memberships are managed under the Dashboard > MemberPress > Products menu. If this menu item is missing, make sure that the admin user has the “remove_users” capability. This is the capability that MemberPress uses to identify administrators. You can use a plugin like Members to view and update user role permissions.
Membership Edit Screen Is Blank
This can happen on MemberPress versions before 1.9.40. Update MemberPress, or make sure the admin user is not subscribed to any memberships.
Meta Boxes Are Not Visible on the Edit Screen
On the membership edit screen, click Screen Options in the upper right-hand corner. Ensure the checkboxes next to the membership product meta boxes are all checked. This needs to be done for each admin user individually.