Depending on the industry that you select when you initially setup you website, there may be service frequencies automatically created for you (eg. One time, weekly, etc.). If you would like to change, add to, or delete these frequencies, you can do this by clicking on Invoicing  Discounts.

Within this page, click on the "Frequencies" button.

Here, you  can change the title of the frequency, change the order that the frequency appears on your booking form, add frequencies, or delete frequencies.