Open the session and click Edit. Changes you make there apply only to that session, not to the activity type or its other sessions.
How to edit a session
Go to Calendar → Schedule and click the session.
Click Edit.
On Edit activity session, open Players, Prices or Settings.
Make your changes and save.
What you can change
Players: change an unpaid player's payment (Marked as paid or Free of charge) or Remove a player with Full refund, Partial refund or No refund. Full refund is pre-selected if the player can be refunded, so check it before you confirm.
Prices: turn off Inherit pricing from activity type to set prices for this session.
Settings: coaches, maximum and minimum spaces, automatic cancellation and registration windows.
Good to know
To add players, use Add player in the session in Calendar → Schedule.
To change courts, open the session in the schedule and use Courts → Edit.
A setting you leave the same as the type keeps following the type. One you change here stays as you set it.
For a course, edit the first session. Its settings (except prices) apply to the other sessions.