Payers can manage their recurring plans within the Payer Portal. Currently, when you create a form, the only way for payers to choose their own frequency at checkout is if you set the form frequency to be "Decided at Checkout." If your form has this setting, you can allow your payers to change the frequency of their recurring plan:
1) Click on the Account drop down button in your account's main navigation
2) Click on the Payer Portal option
3) Scroll to the “Allow payer to” section and click Yes in the “Update plan frequency?” section
4) Click Save
5) Now, when your payers go to their Payer Portal, they will be able to update the frequency of their recurring plan, assuming the settings on the form they checked out on are set to "Decided at Checkout."