How to upgrade OS 10.10.5 to Mojave
Can my MacAir (mid-2012) with Yosemite 10.10.5 upgrade to Mojave?
If it is a mid 2012 MacBook Air, you can upgrade tp Mojave.
How to upgrade to macOS Mojave - Apple Support
Check to make sure your applications are compatible.
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.
