Updating Mac from OS X Yosemite 10.10.5

I cant update my Mcbook Air 10.10.5 early 2014

• Make sure that you have enough storage available for a software update. You'll need at least 8 to 22 GB of free storage space to upgrade to a newer version of OS X or macOS.
• Check what version of OS X or macOS your Mac supports. The latest version of macOS, macOS Catalina, is supported on any Mac introduced in 2012 or newer. If your Mac doesn't support macOS Catalina, it might be supported by macOS High Sierra, which requires a Mac introduced in 2009 or 2010. If your Mac isn't supported still, you could still upgrade to OS X El Capitan.
• Download the update from the App Store. The Mac App Store is the best, and usually only, place to download macOS updates. Be careful downloading update files from other sources.

You can download macOS Catalina 10.15.3 here: Get macOS Catalina

You can download macOS High Sierra 10.13.6 here: Get macOS High Sierra

You can download OS X El Capitan 10.11.6 here: Download OS X El Capitan

You can learn more about how to upgrade to macOS Catalina here: How to upgrade to macOS Catalina - Apple Support

Use the software update in the App Store to download Mojave or even Catalina.

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

your 2014 MacBook Air can upgrade to all these below read the following links.

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.

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