Generally no separate action is required for apps when upgrading the version of macOS. All the installed apps and user data files continue to remain available post upgradation.

However, what's important to keep in mind is to ensure that the app that you are using and wish to continue using is supported in the upgraded version of the operating system before actually performing the upgrade. You can get this information from the vendor/developer of the said app. There may be cases when upgrading the OS will also require you to upgrade the version of the app and associated licensing costs. These all questions could be addressed by the app vendor/developer.

On an important note, Apple also advises to perform a system backup to guard against unforeseen eventualities where the upgrade goes wrong.

I'd also check what's the latest version of OS X/macOS the machine can run - it might not go all the way to High Sierra if it's too old - & plan your app update strategy accordingly.

You should test restoring your system once to be sure you can trust your backup, since people really don't want a backup - they want to know they can restore and the restore works. The backup is just the preparation to do the restore. You can run an erase / install and just reinstall the current OS to test things without the new OS potentially breaking things - that way you know you can restore the current OS first before you even think of upgrading.

- [How to reinstall macOS](https://support.apple.com/HT204904 )

I need to emphasize the **should test** since 10.6 is really a long way back and it's very likely many of the apps will fail on 10.12 or later and need license keys, upgrades, etc... Having a bootable backup or clone might even be worth the effort. You are about to break lots of things and gambling that they are all minor or not used since tons of functions and features from 10.6 are changed / deprecated / turned off now. Any one upgrade alone, generally is easier to risk without a test run, but this is decade of neglect in terms of keeping things moving on the routine update / upgrade chain.