1st Step
Set up the pass.
In Products > Passes > Select the pass that you want to modify
Then, click on "Extend the validity for all members".
2nd Step
Define the extension
Chose the passes to extend, depending on their current expiration dates
Chose the period of the extension
Add some notes if necessary
You'll find the list of the extensions done per each pass available on the right-hand side of the screen.
