Q: Upgrading OS X

Depending on your Mac, you can maybe to to Lion, and have to stop there

Or you might be able to go to Lion, Mountain Lion, El Capitan, and then have to stop

Or you are new enough to go to Lion, Mountain Lion, El Capitan or Sierra

Lion <System requirements for OS X Lion and Mac OS X v10.6 - Apple Support>

Mountain Lion <OS X Mountain Lion system requirements - Apple Support>

El Capitan <OS X El Capitan - Technical Specifications> Get El Capitan via <https://support.apple.com/en-us/HT206886>

Sierra <https://en.wikipedia.org/wiki/MacOS_Sierra>

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.      

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

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.