It very much depends on your system set up. Start with a read of https://confluence.atlassian.com/adminjiraserver071/upgrading-jira-applications-802592251.html
I strongly recommend a two-step upgrade. Either 6.2 -> 6.4 -> 7.3, or more likely, as I've had issues with direct upgrades to 7.2, 6.2 -> 7.0 -> 7.3. Test the route that seems best to you before deciding, and plan for the worst case of 6.2 -> 6.4 -> 7.0 -> 7.3
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