You may have two accounts linked to you Measured account at anytime. One account will be deemed your primary account and will used to disburse draws to and if you have automatic payments set up, funds will be pulled from the primary account when your payment becomes due.
When making a one-time payment, you will be able to select the account to use in order to make the payment.
To change payment methods, simply log into your Measured Dashboard, navigate to "Settings" by clicking on the person icon on the top right hand side of your dashboard to open the drop down menu. Navigate to "Accounts" tab on the left hand side. You will then be able to link an account or unlink an account.