You can cancel your plan at any time through your Helium Mobile account, from within the Helium Mobile App, or by contacting Helium Mobile Support.
The full cancellation policy can be viewed in the Terms & Conditions (Section 21)
When will my Helium Mobile service end after cancellation?
I canceled my plan and requested to transfer my phone number.
Your Helium Mobile service will continue until your phone number is successfully transferred to another carrier.
I canceled my plan and did NOT request to transfer my phone number.
Your Helium Mobile service will continue until the end of your billing cycle.