how to update OS X version 10.14 on my apple lop top macbook air

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation.

Back up your data, check your applications for compatibility, and if your MacBook Air is a 2012 or newer model, click on Mojave. Otherwise, you can't.

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Hello, iIela65.

Which MacBook Air?

Please tell us a bit more about your Macintosh. Which model and model year is the computer and what macOS version are you running?

If you are able, click the Apple () menu > About This Mac.  There you will find the specifics we can use to help you best.

(Note- for privacy and security please do not post your serial number.)