Extraído da página oficial do Xbox Live :
To switch to a different Microsoft account than the one you’re signed in with in your Xbox app—as you’d want to do if your Xbox gamertag is associated with a different Microsoft account (formerly known as a Windows Live ID)—do the following.
Sign in to your PC using a different Microsoft account
On your PC, select the Settings charm. Select More PC settings. Select Users. Under Other users, select an account.
If needed, select Add a user and enter the email address and password for the Microsoft account you want to use.