You can switch from your monthly Unity Pro plan to an annual plan online through the Unity ID portal. Log into your Unity account to get started, then follow these illustrated steps.
Select the organization you want to change
From the list of Organizations affiliated with your account, select the name of the Organization that currently has one or more monthly Unity Pro subscriptions.
Modify your subscription
In the detail view for your Organization, under Subscriptions, click the Save with an annual plan button in the Unity monthly plan field.
Choose an annual plan
Click the Save with an annual plan tab.
Choose your payment frequency
Select whether you want to pay monthly or prepay for the year. Click Accept.
Review and confirm your purchase
Click Confirm Purchase to complete your transaction.