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
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