Go to your Profile page → Billing. If you have an active Plus or Creator subscription, you'll see an "AI Assistant add-on" row with a Turn on / Turn off button.
When the change takes effect
Turning the add-on on or off for an existing subscription never changes today's bill. The change is scheduled to take effect at your next renewal date instead, you'll see a confirmation naming that exact date. Until then, your current add-on status (on or off) keeps working exactly as it did before you clicked the button.
Changed your mind before the change takes effect? Click the button again, this cancels the pending change and keeps things exactly as they currently are, no extra step needed.
Adding it at signup instead
If you're subscribing to Plus or Creator for the first time, you can check "Add AI Assistant" right on the Pricing page before checkout, it's then included from day one instead of being scheduled for later.
If you cancel your whole subscription
Cancelling your Plus/Creator subscription also turns off the AI Assistant add-on automatically once the cancellation takes effect, you don't need to separately turn the add-on off first.