Yes, you can absolutely upgrade yourself.
Probably best first read through the migration hub documentation https://confluence.atlassian.com/migration/jira-7?tab:platform=1
Since JIRA has been split into 3 different applications you'll need to decide which of the 3 you will end up installing as the 'latest' JIRA version. (or alternatively, upgrade to the latest 6.4 release and then work out what to do about JIRA 7)
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