Just go to Xbox store and go to warmind dlc if you already own the the destiny 2 season pass go search in the Xbox store destiny 2 warmind and it should be free just donwlaond it even if you already have it in-game you still need to install it
Hope it helps
(Ask me anything if you don't understand)
Yes but there is a glitch in the store it says that you own it but you do not have it installed but I'm ps4 so I'm not sure about Xbox I was just trying to help you