Upgrading mac book air

A MacBook Air (13-inch, Early 2015) can be upgraded all the way to Monterey, but no later.

How to download and install macOS - Apple Support

Because Yosemite is so old, it is possible that it will have trouble accessing the App Store, and that you will not be able to perform the upgrade in one step. You may want to download and install Sierra, and then do the upgrade to Monterey using the newer versions of Safari and the App Store in Sierra. When using App Store links in the article, be sure to have Safari set as your default browser and to be using Safari. Using other browsers (like Firefox) might result in very misleading error messages.

I don't know whether your MacBook Air has 4 GB or 8 GB of RAM. Hopefully it has 8 GB. The RAM is soldered in; if you have only 4 GB, I think you could still upgrade to Monterey, but you might have trouble meeting the minimum system requirements for many newer applications.

Note that if you upgrade to Catalina or later (Catalina, Big Sur, Monterey),

• 32-bit applications will stop working. Since you are using Yosemite, it is very likely that you have some of these. You can use the Applications tab in System Report ( > About This Mac > System Report…) to identify these. Wait for the Mac to list the applications, then click on the 64-bit (Intel) column header, to sort by values in that column and group all of the 32-bit applications together.
• iTunes will go away, replaced by separate apps: What happened to iTunes? - Apple Support

What MacBook Air do you have? The age of your MacBook Air will determine how far you can upgrade.

You should be able to find this in the About This Mac dialog in the Apple () menu. Model name, screen size, and hardware year / time of year only, please – we don't need the serial number.