How do I upgrade my Yosmite to the latest os ?

Here's how to update your laptop to the latest software version:

Mid 2012 MacBook Airs support all versions of macOS up to macOS Catalina (10.15).

To update:

1. Create a backup. This will ensure that if something goes wrong during the update, you will not lose any data, and you will be able to restore to your previous machine state.
2. From the  menu on your menu bar, choose "System Preferences"
3. Select "Software update"
4. Check for any available updates. You should see that macOS Catalina is either available to be downloaded or installed.
5. Click on "Update/Upgrade Now"

For more information on MacOS Catalina, the devices that support it, and technical specifications:

macOS Catalina - Apple (UG)

macOS Catalina - Technical Specifications (UK)

For more information on backing up your Mac using Time Machine or other methods:

Back up your Mac with Time Machine - Apple Support

Other ways to back up and restore files – Apple Support (UK)

For more information on updating your Mac and troubleshooting:

Update macOS on Mac - Apple Support

macOS - How to Upgrade - Apple (UG)

If an error occurred while updating or installing macOS - Apple Support