How to Approve Mac on iCloud from iOS Device
Click Setting on iOS device > Choose iCloud > Enter iCloud ID and password. Get Apple ID Verification Code from another iPhone, and enter the 6 figures. The only difference from approving Mac on iCloud is that after you enter Apple ID Verification Code, you will be asked to enter the iPhone passcode, too.
Just open your iPhone which is signed in to iCloud and it will tell you there is another device on that iCloud and it will ask you to allow it or not. Click allow and it will give you the Apple ID Verification Code. Enter the Apple ID Verification Code and you have successfully approved Mac on iCloud.
How to Approve iPhone/iPad from Another Device
With two-factor authentication, your device is trusted when you sign in with your Apple ID, so in most cases you don't need to re-approve it. On your iPhone, iPad, or iPod touch: Tap Settings > [your name] > iCloud. For 10.2 or earlier, tap Settings > iCloud.
If your computer is online:
There is no way to do this from a Windows PC. A code won't be sent to a non Apple Device/Computer.
How to approve iPhone from other non-Apple devices(PC, Windows/Linux device)
Now you know how to approve iPhone on Mac and other Apple gadgets.If your computer is online:
From one of your iOS devices, go to Keychain and tap Approve with Security Code. When prompted for the 4-digit iCloud Security code tap Forgot Code. Follow the prompts to reset the code (the easiest thing is to use the same code that you use for the device you're on).
From your iPhone, go to Keychain and tap Approve with Security Code. When prompted for the 4-digit iCloud Security code tap Forgot Code. Follow the prompts to reset the code. Now this device should have iCloud Keychain enabled and working.
Question: Q: How do I approve my iphone on another device it is telling me to after an update