This will change whatever you use as your default UW email address.  Be aware that changing this address will change what email is listed as your reply-to in any email communications through the UW.

Go to https://email.wisc.edu/admin.  

Login with your NetID.

Click My Account.
Click Office 365.
Click Email Addresses.
Choose which email address you want to use as your outgoing email address.

You should ALSO change your email address that HR considers to be your default.  

Here's how:  https://kb.wisc.edu/page.php?id=18559