If your calendar is on our free plan, you can test out a paid subscription with a free trial:
Open your calendar in a browser with administrator access.
Go to Settings > Subscription.
Click Start Free Trial for the subscription level you want to test.
Each trial period lasts for three days.
If you want to start the subscription during a trial period, add payment details. The charge will go through immediately and your paid subscription period will begin.
After the allotted three days the calendar will automatically return to the free plan unless you have entered payment details. The free plan can be used indefinitely.