Yes, of course, you can edit them by clicking on the 3 dots that you see on the right side for each membership you have created. Moreover, the edits will only impact new members. Any existing members will not have the edits - meaning if "XYZ" signed up for Membership A with 2 services per month and then it was edited to 10 services per month after "XYZ" signed up, "XYZ" would still only have 2 services per month.
Q. Will editing a membership plan affect the existing members and their subscription details?
A. No, it will not affect the existing members and their subscription details.