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 'Edit Trial End Date' 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 'Edit Trial End Date' option:

3. Enter the new Trial End Date & update.

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

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