You can modify your subscription by logging into your BURST account on the BURST website.
You can log in through https://www.burstoralcare.com/login.
Modifying your subscription from a Laptop/Desktop
Once logged in you will see the person icon 
in the top Right-hand corner, This will bring you to your account and you can then select my Subscriptions tab from here you can edit your subscriptions with BURST.
Modifying your subscription from a Mobile device
Once logged in you will go back to main page, Select the Menu from the top left-hand side, and account.
This will bring up your account and you can view the My Subscriptions tab and from here you can edit your subscriptions with BURST.