Search for the customer you want to amend the account code for
Click the Accounts Codes tab
In the grid, select the account code that you want to add the agreement to. If the account code doesn't exist yet you can create a new one using the Create Account Code button.
See How to: Add an Account Code
Click the Add Agreement button
In the popup, search for the agreement you want to add to this account code
Click the Ok button
The selected agreement is now allocated to this account code