How to upgrade legendary items to ilvl 940 (World of Warcraft: Legion)

It is possible to upgrade any legendary item to ilvl 940, and this is done by using an item called Distilled Titan Essence. To get this, you need to pick up the quest Touch of a Titan from Aman'Thul in Dalaran. He will need you to collect 50 x Essence of Aman'Thul. These essences drop from bosses in Nighthold, from Mythic Keystone caches, Emissary bags and the weekly PvP quests. There is no cap on how many you can get weekly, so do all of these to get as many as possible.

Once you have 50 essences, head back to Dalaran to turn it in, and use the item on the desired legendary to upgrade it to ilvl 940 permanently.