Hi, there are two ways to accomplish what you are trying to do.
Option 1: Associate the account that is signing in currently with a Microsoft Account and then disassociate the account in the users menu in Settings. This will free the user up to sign-in with another xbox account on next launch.
Option 2: Registry editing (see steps to do this below).
Registry Editing (Option 2)
1. Open a Run Prompt (windows key + R)
2. Type “regedit” (without the quotes)
Navigate to this location: \HKEY_CURRENT_USER\Software\Microsoft\IdentityCRL\Immersive\Production
3. Delete the entries there
4. The next time the user launches the app, the user will be prompted to sign-in again
Please reply and let us know if these methods worked for you! :)