can I upgrade my OS from my macbook air 11" 2011 version from OSX Yosemite to a more current OS

The model and year of your Mac will determine which macOS you can upgrade to,

Please use Safari to access the following links do not use Firefox

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to OS X El Capitan - Apple Support

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. 

On your version of macOS, it looks like you'll need to update to El Capitan 10.11 first. From there you can more swiftly access the App Store and further upgrade to the current Catalina if your system supports it.

Here is a support article with more information: https://www.apple.com/in/macos/how-to-upgrade/

Please be sure to make a full backup of your current hard drive. (Either with Time Machine, or a different service of your choosing.)

In step 4 of this article, there is a download link for osX 10.11 https://support.apple.com/en-us/HT206886