You can't. You have to create a 3rd WLID that you dont' care about, account.live.com/signup and attach to it the gamertag that has the email you want, has to be done on the console, then take the good email and switch it with the good gamertag.
Rules for changing the WLID to a gamertag:
If the password has been reset or the profile downloaded in the last 30 days you can't change the WLID on a gamertag.
If you don't know the password to the gamertag and that's why your trying to change the email that's not going to work. You must first do account recovery, then the process above.
To change the WLID on the console:
Sign in, go to Settings, Account, arrow over to the right twice to "Account Security" and "Change Microsoft Account"
Then enter in the current password for the account, then the email your switching it too, then that password.
Again if you've downloaded the profile or reset the password in the last 30 days you'll get an error message or if it's already attached to another gamertag.
I AM NOT A MICROSOFT EMPLOYEE NOR FORUM MODERATOR. However I do know a lot about acct recovery.
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