Jessia - I manage at on the customer record. Go to Customer Details fly out, edit the Tags, scroll to the bottom, use the Payment Terms drop down to select the term, select Save. Note the term you indicated will apply only to invoices for jobs created after you set this up. Until you bill out all of those jobs you will need to manage on each individual invoice using the Update payment terms & due date on the left when you are sending out invoices.
Note you can define/set up payment terms in settings.
Hope this was helpful
