Unable to update ISO on Mac

If you are referring to an ISO file (.iso), which is a type of disk image file, and you are trying to mount that, post back with more details. A .iso file usually a copy of a physical disc (CD-ROM, etc.), and thus cannot usually be updated or modified.

I presume by ISO that you mean iOS, and since you reference a Mac you really mean macOS.

First check if your Mac is compatible with macOS 27 Golden Gate. That will depend on the specific model. Apple’s current compatibility list is here:

macOS 27 Golden Gate is compatible with these computers - Apple Support

If you are not offered an update in System Settings and/or are unable to install an update downloaded via Terminal, then is likely that you are in Low data mode. Users have reported difficulty upgrading macOS over Wi-Fi in Low data mode, and that toggling that setting OFF enables them to perform a Software Update. 

Turn off Low data mode

• Open System Settings 
• Click on Wi-Fi
• Click on the Details button on the right side of your connected network
• Toggle the setting off

More info from Apple: Wi-Fi settings on Mac - Apple Support

If macOS 27.0.1 does not appear in Software Update, try the following steps in order.

1. If you are using a VPN, shut it off.
2. Restart your Mac.
3. Go to System Settings > General > Software Update and check again. Press Command-R to refresh that settings pane, just in case.

If macOS 27 still does not appear, you can check which software updates are currently being offered to your Mac.

Open Terminal (find the app with Spotlight or in Applications > Utilities) and paste the following, then press return:

softwareupdate --list-full-installers

If macOS 27.0.1 appears in that list, you can download the full installer with:

softwareupdate --fetch-full-installer --full-installer-version 27.0.1

Why?

What happens when you try exactly? Do you get an error? If so, what does it say?

Can you post a screenshot of the Software update screen in System Settings ➜ General ➜ Software Update.

Also, if you can tell us what model year of Macbook Air you have, it can help us determine if it's compatible with macOS 27 Golden Gate.

click here ➜ Update macOS on Mac - Apple Support

First check if your MacBook Air is one of the models compatible with Golden Gate.

macOS 27 Golden Gate is compatible with these computers - Apple Support