If the XBox account needs to be replaced with a different XBox account, then please log in to bungie.net using your Playstation credentials. Once logged in click on your player avatar in the top right of the web site, then select Settings. On the Settings page there is a sub-menu called Accounts and Linking, click on that. You should see a red unlink button next to the XBox account. Please click that button, enter this XBox account's credentials to remove it. Once removed, on the same page you can now link a new XBox account.
Did you already deactivate Cross Save? If so, you will need to wait out the 90-day cooldown period, as Bungie does not cancel or void this for security reasons.