Update to os x 10.10 (from os x 10.9.5)

If you only want to upgrade to Mac OS X Yosemite you can, Apple recently re allowed it for download

read Sections 4 to 7 in the link below for instructions on how to download and install,

How to upgrade to OS X Yosemite – Apple Support

Other more modern OS's are available for install for your MacBook Air 2014,

macOS Catalina 10.15 was released recently, the latest version of macOS.

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

If you want to keep running your 32bit apps while you ponder moving up to Catalina

then there are other OS's you can choose to upgrade to. Mojave is macOS 10.14,

High Sierra is macOS 10.13 and El Capitan is Mac OS X 10.11.

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.

Back up your data, check your applications for compatibility, and download Catalina.

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