If you would like to switch from being billed monthly to being billed yearly or vice versa you can do so via the Settings > Workspace > 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. 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.