upgrades

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Check to make sure your applications are compatible.        

Application Compatibility

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

It depends on the year of your Mac. (See Apple menu > About this Mac)

macOS - How to Upgrade - Apple