Cloud version:
For cloud apps, you cannot extend your free evaluation period. All cloud apps are immediately subscribed by a user, and we provide a free evaluation period. This is a minimum of 30 days and ends on the second billing cycle after you first subscribe to the app.
Data Center version:
For Data Center 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 —
Click ‘Try it free’ and you'll be directed to generate a new license.
Paste this license key into your Atlassian product's app listing in UPM, and you're all set.