I have a macbook air with Yosemite 10.10.5 I cannot get it to upgrade.

What model year is it?

The model year of a MacBook Air is available at Apple menu > About This Mac.

A late-2010 through mid-2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version.

A mid-2012 MacBook Air can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A mid-2013 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475

Which OS you can upgrade to will be governed by which model and year your mac is.

A 2015 MacBook Air can upgrade to all available OS's including the latest macOS Big Sur 11.4

If you want to upgrade to any of the newer OS's like macOS Mojave 10.14, macOS Catalina 10.15, 

macOs Big Sur 11.4 I would recommend you upgrade to macOS High Sierra 10.13 first.

For older OS’s,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.

Then go to the Download OS section and click on the OS you want to install.

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 want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do 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/ then contact the developers of the software to see

if they are offering 64bit updates.

An early 2015 should upgrade to Big Sur using the following instructions How to upgrade to macOS Big Sur - Apple Support but as you noted, there are free space requirements. You will need a minimum of 44.5GB. See the following from the above instructions:

If upgrading from macOS Sierra or later, your Mac needs 35.5GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44.5GB of available storage. 

Golf_Mom_Of_2 wrote:

I have the following model: MacBook Air (13-inch, Early 2014)

I have been trying to update and I can't get get any other OS to update or get any additional updates for OS X Yosemite past this version.

What happens when you use the following instructions? How to upgrade to macOS Big Sur - Apple Support

I have the following model: MacBook Air (13-inch, Early 2014)

I have been trying to update and I can't get get any other OS to update or get any additional updates for OS X Yosemite past this version.

As far as I know my Grandma's macbook does the same and she has cantina and it won't upgrade to big surr because of her storage.

What model year is it?

The model year of a MacBook Air is available at Apple menu > About This Mac.

All the instructions have been given above, read them and follow them.