For server apps, you can extend your app trial up to 5 times - in other words, for up to six months. 
Extend your trial by generating a new evaluation license key from Atlassian Marketplace.

Follow these steps

  1. Navigate to Manage Add-ons in the JIRA Administration.

  2. Find the Agile User Story Mapping for Jira plugin.

  3. Delete the existing licence key.

  4. Click the Free Trial button.

You should now have a new trial license for 30 days.