Within Teamwork Desk you can change your subscription plan at any time.
To do this, click on your profile icon to the top right of Desk and choose 'Settings' from the dropdown menu:
In the settings area, go to the Billing tab:
In the subscription tab of the billing area, you will see a section for 'Billing Usage Details.' Below this there is an option to 'Update Subscription.'
When you click update, you will be brought to the checkout where you can adjust the number of users on your Desk account, choose a different plan or switch between monthly and annual billing:
Below the plan options, you will also be able to adjust the number of help docs sites on your subscription:
For more information see: How billing works