You need to perform an intermediate upgrade to 7.0.0.
Before starting to work on upgrade, go through the Upgrade notes and release notes for all versions from 6.4.x to 7.4.x and summarize it.
Upgrade notes will tell you if there is any particular to look out for. Release notes makes you aware of any major changes that you need to plan for.
List the plugins that you currently have. Most important is non atlassian developed plugins. If any of that has any problems then that you might find in release notes.
Make an upgrade plan which will list every single step that you want to perform during the upgrade and the time for that action.
Do a test environment upgrade first and make notes for important observations and make changes to the plan accordingly.
For database, you can create couple of empty databases (same config as your current database) on the same server as standby databases and dump the production data on those databases when you take JIRA down for upgrade. You can use one of those databases in case of rollback scenario since you cant downgrade existing database.
For installation directory back-up, you can simply use copy of Jira Home and Installation directory into one directory eg 'atlassian' directory where Jira home and installtion directory is you can copy that to 'atlassian_backup' and simply rename it in case of rollback.
Make an post-upgrade checklist for issues, attachments, icons, avatars, all plugin functionalities, Jira properties, database collation, Lexo rank, health check, custom attributes in important files like setenv.sh, server.xml, web.xml, jira-config.properties etc..
Hope this helps. Good Luck :)
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