Change email apple id

Did you use that email@outlook.com AppleID to make your iCloud account? If so, the email@icloud.com address is an alternative email to that same AppleID.

In other words, if you make an AppleID with a third party email address, and then make an iCloud account using that AppleID for the initial sign-in, then both the third party email address and the iCloud email address are equivalent logins for the one AppleID.

I can sign into my Appleid using my primary (and third party email address), or my iCloud email address from that AppleID, or my legacy @me.com address that is an alias to the iCloud email. They all work to sign into the one AppleID tied to them all. But since the iCloud email is tied to that AppleID (and is itself a functional AppleID email address), I can’t replace my third party email address with that iCloud address - that’s not allowed.

Based on what I have read, I suggest you leave your Apple ID email address as a 3rd party email address because you have more flexibility if you need to change the email address. For example, if the Apple ID got compromised, you can change it to another address. I also have read of many people who were locked out of their accounts when they forgot their password. Since the recovery email address is the @iCloud email address, if you forget your password, you also can't access your email to get emails from Apple to recover your account. You can set a rescue email address, but for some people, this hasn't worked.

Why can't you update it? What happens when you try? Error message(s)?

Note the restrictions in the article.

Apple ID Name Change

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.