This article also answers:

Is it possible to change the trial end date using the APIs?

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You can either extend or shorten the trial period for a subscription using the 'Change Trial End' option in the web interface. Here’s how to do it:

 
1. Open the subscription for which you would want to change the trial period.

 2. From the right panel, select the 'Change Trial End' option as shown:

3. Enter the New Trial End Date.

4. You’ve now successfully changed the trial end date of a subscription!

You can use  Change Term End API if you're an API user.