You can cancel your Maps Engine Pro subscription at any time using Google Wallet (get instructions). Because you pre-pay for service and receive a 7-day grace period for cancellations, your subscription will end 7 days after the next monthly renewal date. Once your subscription ends:
You will lose access to Maps Engine Pro, and your account will revert to Maps Engine Lite.
All of your data will remain in your account, and you can still access your maps.
Any maps you shared publicly will remain publicly accessible, unless you change their sharing option.
You won't be able to edit any maps with Pro features (such as layers over the Maps Engine Lite limit), unless you re-activate your Pro subscription.