You keep access to everything until the end of the period you already paid for. No reason to give, no email to send, no notice period.
Until the current period ends, a Reactivate button appears in the same place and restarts the subscription with no new payment. After that date, you'll need to pick a plan again.
Same tab, Plans section: pick another one. Stripe prorates what's left of the current period.
Use the contact form from your account address: we cancel by hand, the same day.