Question: gow how to upgrade blades of chaos?
Answer: Unlock Muspelheim (Find all 4 cipher chests.)
Complete Trial 1-5 and reach the Valkyrie at the top.
Defeat the Valkyrie — she drops the item you need to trade at Brok / Sindri's shop.
Trade the item in to get the final upgrade item for the Blades of Chaos.