If you no longer want to continue a recurring ChatGPT Plus subscription, cancel it before the next billing date.
Cancel ChatGPT Plus on the Web
The web flow at the time of writing was:
Sign in to ChatGPT, select the account avatar, and choose
My plan.
Select
Manage my subscription.
On the current-plan page, select
Cancel subscription.
Confirm the cancellation.

The interface may change over time. A survey may appear after cancellation; completing it is optional.