1) Head on over to https://www.bandainamcoent.com/ and click "Log In" at the upper-right hand side of the page.
Figure 1: BANDAI NAMCO Entertainment America Home Page
2) Enter your Bandai Namco Entertainment Account credentials and click the yellow "Log In" button.
Figure 2: Bandai Namco Account login page
3) Once signed in, click on "My Account" in the top right hand side of the Bandai Namco Entertainment America home page.
Figure 3: Bandai Namco Entertainment home page while signed in
4) On your My Settings page, you'll find a short list of options on the left side of the page. Click on "Account Linking".
Figure 4: Account Settings page
5) On the Account Linking page, you'll be able to manage your BANDAI NAMCO Entertainment America Account links with Xbox Live and Twitch. Click the "Unlink Account" button found under the Xbox Live section.
After that, you should see a notification pop up at the top right hand corner of the screen confirming that your accounts have successfully been unlinked!
Figure 5: BANDAI NAMCO Entertainment America Account Linking Options
Figure 6: Account Unlinked Confirmation