For members to be able to update their reservations from their member portal page, you'll first to need activate this under Booking Settings from the admin side.
We'll go to Settings > Booking Settings > General
From here, we will scroll down until we see these boxes:
Note: 'Don't allow reservation edit if less than X hours till Reservation Start' is a great setting to prevent potential court hogs/people trying to game the system.
For a player to update their reservation, they'll just go to the member portal page, click My ACCOUNT > MY RESERVATIONS