Go to my.atlassian.com and select Licenses. Find the Mobility for Jira add-on then click on New Evaluation License (see screenshot). Copy/Paste the new evaluation key into Jira/Manage Add-ons/Mobility for Jira
You should be able to extend it up to 3 times. If that option isn't there, just open up a ticket at support.atlassian.com. They will be able to extend it. It may take about 24-48 hours.
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