If you would like to switch from being billed monthly to being billed yearly or vice versa you can do so via the Settings > Plan & Billing area of the Timeular app

In the Your plan summary to the right of the "Payment" header there will be a link that will allow you to switch your payment frequency:

Just click this link and the change will be made. You'll see the summary is updated once it's successful:

And that's it, you've changed your billing frequency. You'll receive an email with the receipt and invoice for the transaction shortly after you've made the switch.