You can do that by clicking the Account Mapping option in your ePayments profile. This opens the Account Mapping dialog where you can map your AffiniPay accounts (e.g., merchant account or ACH/eCheck account) to a corresponding Core account. The mapped accounts are then available in the Deposit Account drop-down in the profile. The Account Mapping dialog is accessible from all profiles and you can change the mappings at any time if you want. Check the Core Help for more details.