You need armor with light 42. Old Exotics can be upgraded to light 42 with an exotic shard, and new exotics(ones you've gotten after the HoW DLC started) are already light 42 or won't require a shard to upgrade but you may still need to compile the upgrade tree. To upgrade your legendary armor from 36 to 42 you need to use etheric light on the armor. You can also buy armor from varkis which is already at light 42.