The invoice period for an agreement will affect how often the agreement is invoiced, changing the invoice period will also change the service from and service to dates for the agreement accordingly.

For more information see Understanding Invoice Periods. 

To Change From Agreements screen

1. Search for your agreement from the Agreements screen.
2. Click Options then Amend Invoice Period.
3. In the popup, select your new invoice period; you can also select a new invoice date if required.
4. Once you've selected your new details, click Ok.
5. The agreements screen will refresh, showing your new service from and service to dates and the invoice period will have changed.