To upgrade to the Scale plan, just click on the Scale plan in billing.
You will then be taken to stripe where you can enter a credit card to be charged.Alternatively, you can choose to pay through AWS Marketplace. See instructions here.
To cancel your Metoro membership, move yourself to the hobby tier by clicking on the plan in billing.
When you move to the hobby plan, you will immediately be charged for the amount used in the current billing cycle on your existing plan. No further charges will be made.