You may cancel your oDASH subscription at any point of time from within your oDASH account. Your subscription gets marked as Cancelled but the Premium / Premium+ plan stays on till the date you have already paid for. Post that, your account downgrades to Basic plan.
To cancel your subscription, login to your oDASH account and click on Subscription button on the header. You will get re-directed to the page which will show Cancel button.