Changing or cancelling your plan
Any time. Access runs to the end of the period you paid for.
To upgrade, pick the higher plan in the plans window. You pay the difference for the rest of the period and get the extra credits at once.
"Manage subscription" on your account page updates your card, shows invoices, or cancels. To move down, cancel and pick the lower plan when the current one ends.
Or use Leaving → Cancel plan. Access runs to the end of the paid period, then the account moves to Free, keeping your library (within Free's storage) and bought credits.
On the account page we ask why you're leaving. Answering is optional and Cancel plan works either way.