Payers can manage their recurring plans within the Payer Portal. To allow them (within the Payer Portal) to update the custom field responses they provided at checkout:
1) Click on the Payer Portal option in your Account drop down menu
2) Scroll to the “Allow payer to” section and click to enable the “Update custom field responses?” option
3) Click Save
4) Now, when your payers go to their Payer Portal, they will be able to update their custom field responses.