You can switch between monthly, semiannual and annual billing from diio settings. The annual plan includes a discount compared to the monthly price.
Go to diio settings → Billing.
Find the "Choose your payment plan" section.
Select the new frequency with the radio buttons: Monthly, Semiannual or Annual.
The system shows the price with the new frequency and the applicable discount (if you switch to Annual).
Confirm. A prorated charge for the change is made immediately.