Can I update from Yosemite to Catalina?

Only 2012 or later MacBook Airs can update to Catalina. See: How to upgrade to macOS Catalina - Apple Support

Late 2010 to 2012 models can update to High Sierra. See: How to upgrade to macOS High Sierra - Apple Support

You can see the model year at Apple menu > About this Mac.

It might be better to upgrade first to macOS High Sierra and from there upgrade to Catalina.

It has been noted that the install of Catalina can often stop if in stalling from older OS's like Yosemite

as the installer is trying to convert the older HFS+ file system to the newer APFS file system.

Installing to High Sierra first may make this transition easier.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support