Updated for 7.0
Click on the Settings icon (the gear) located to the right of the search bar. This will take you to this page:
In the Accounting box, select Payment Types. This will take you to this screen:
To edit, Click on the box of the desired payment type.
To make a payment type active/inactive, click the on/off toggle to the right of the name.
To add a payment type, click the + Add button in the bottom right-hand corner.
When done, click the Update button in the bottom right-hand corner to save changes.