Howdy Paul!

Certain payment processors, such as Authorize.net, allow you to put an account into test mode within NationBuilder. This can be done by an Admin under Settings > Bank Accounts > All Accounts > Edit.

Other payment processors, such as Paypal Payflow Pro, do no allow editing of test mode in NationBuilder. In this circumstances you will need to refer to the documentation of the payment provider and how it can be changed within your account settings on their site.