Via the dashboard — effective from the next billing period. No notice, no exit fees.
You cancel via the dashboard — there's a button under "Account and subscription". The cancellation takes effect from the next billing period, and you keep full access through the period you've already paid for. There's no notice period and no exit fees. If you've prepaid for a full year, you cancel the same way, but the remaining amount isn't refunded (you keep access through the period). If you want to keep your data before the account is closed, export it first — see the export question in the same category.