You can cancel at any time. Cancelling stops the next renewal but doesn't cut off access immediately — you keep watching until the end of the period you've paid for.
A confirmation email is sent once it's done. After the paid period ends, the account is suspended (not deleted), so you can reactivate later without losing your history.
Free trial: cancel within the first 7 days to avoid being charged. Apple/Google purchases: if you subscribed through the App Store or Google Play, cancel in that app store instead.