There's two parts to this.
The Confluence upgrade stuff, you need to start at https://confluence.atlassian.com/doc/upgrading-confluence-4578.html
The harder part is going to be the OS upgrade. Not because those are hard in themselves, but because they make the Confluence upgrades potentially more complex as you will be changing the foundations Confluence is running on.
So. I would start with a read through the Confluence upgrade doc and work out what versions you need to go through. Then look at the "supported platforms" for each version. Most important is the version of Java.
Hopefully, RH7.5 supports the version of Java you are currently on with Confluence, and I'm certain it will support more recent ones. If this is the case, then your upgrade is a doddle - take a backup, upgrade the OS, then upgrade Confluence.
If it is not that simple, you may need to step through Java versions.
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