What did David use to cut off Goliath's head?

Answer: Goliath's own sword

With no sword of his own, David ran, drew the Philistine's sword from its sheath, killed him, and cut off his head.

“Thus David prevailed over the Philistine with a sling and a stone; without a sword in his hand he struck down the Philistine and killed him.”— 1 Samuel 17:50 (BSB)