This is the upgrade doc: https://confluence.atlassian.com/adminjiraserver071/upgrading-jira-applications-802592251.html
As for costs, if you've been paying for renewing the maintenance and support, there's no additional cost. Otherwise, you'd have to pay for that and all plugins that need to be upgraded. That would ordinarily be 50% of the purchase price at your user tier. You can upgrade to whatever version was released during the maintenance window.
That's all explained in the Pricing FAQ
Over the next several weeks we'll be sharing some of our Getting Started guides here in the community. Throughout this series of posts, we'd love to hear from customers and non-customers ab...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs