If you have a license, it is valid for one year of support (you can raise support tickets) and for upgrade to the latest available version.
But technically, the upgrade is not free because you have paid for it already with your annual license cost :-)
Option 1: If you once purchased on your behalf Jira Software license, it should be visible like on the screen below:
1. Support Expires - license due date, up to this date you can upgrade Jira
2. Request support - if you have a production non 10 user license, you can raise a support tickiet here
More about support can be found here
Option 2: If the license was purchased by someone else but you are an Administrator of your Jira Server, you can check the details of your license here:
Jira Administration / Application / Versions & licenses
G’day Community! As we gear up to introduce Bamboo Data Center to the world, we wanted to make sure that we shared a bit more about Bamboo, the product. Our team has put together an overview video ...
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