2015 Macbook air needs updated IOS but can't download any?

Hello,

Well for starters you can't find an iOS for it because Macs don't run iOS, they run macOS. ;-)

As for why you can't download any, you don't really give us any information to answer that. It would speed assisting you if we knew the details of what you are trying, what you expect to happen, where it is not working, and any error messages you are seeing. In fact your computer with Yosemite can run a whole bunch of newer system versions. Get more information about your computer. Choose Apple menu  > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". More about this in "About System Information [Profiler] on your Mac" - https://support.apple.com/HT203001

You can use the computer information and the links below to yourself determine what is possible with your computer but it can potentially run system versions up to the newest, Big Sur.

Click on this link to read how to get different versions of macOS - https://support.apple.com/HT211683

- This article also has links to system requirements for each OS version which you should check against your exact computer model. Note some of the model identifier articles are outdated. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

- Make sure you have more RAM (memory) than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get links for High Sierra and later using the Mac App Store application to open a functioning page.

- Sierra, El Capitan, and Yosemite download as a .pkg. Click on this to have a converted version placed in your Applications folder as an "Install OS..." application. Run the installer and follow the directions.

- Upgrades may take some time (up to an hour or more) to install, with restarts and blank screens.

Back up your data in case something goes wrong --> Back up your Mac with Time Machine - https://support.apple.com/HT201250 or use a cloning tool.

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/

Hi,

Your MacBook Air is Big Sur supported model. You can download and install from following site.

https://apps.apple.com/us/app/macos-big-sur/id1526878132

Before jumping in and installing Big Sur as recommended in another post, read my other post. If you install Big Sur you may find you have too little RAM for it to run smoothly, plus if you have a bunch of old applications you may be faced with suddenly discovering a bunch of them won't run. I don't know if RAM can be upgraded in your model Mac -- it kind of dates from the generation when everything was soldered in and couln't be upgraded. As for the apps, eventually you will need newer versions but it is nice to know you need to make that investment and perhaps upgrade at a later date after you have saved a couple of hundred dollars. Some people have mission-critical old software with no modern versions and need to accommodate that in their upgrade plans.