To modify the default Terms & Conditions that Buildxact supplies you can do the following.
First of all open any job (doesn't matter which one), then go to the Outgoing Invoices tab.
Click on the cog icon
Click on "Manage Payment Terms".
A new window will appear where you can enter your own Terms. You can also reset to the Buildxact default T&C by clicking the refresh button down the bottom right of the window.