Open the inventory and click the box(es) to the left of the item’s coloured icon. This will show all the upgrades in your inventory for that item.
Maybe I just missed the relevant part of the Inventory tutorial (was there one?) but it wasn’t immediately obvious to me how to use weapon and armour upgrades.
I knew I was looting items I could use and was wondering where they went. After some unsuccessful googling I finally figured it out so I thought it deserved a post.