First, a minor point on versions - they're all "stable" if they're available for download. Atlassian don't release unstable versions (except, arguably, the early access releases)
Start at https://confluence.atlassian.com/adminjiraserver/upgrading-jira-applications-938846936.html for reading on how to upgrade.
In your case though, I would look at it as two separate steps. Jira 8.11 and 8.13 are both supported on Postgres 9.6.15 and 9.6.20. So upgrade Jira and then upgrade the database. Or the other way around.
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