To change the item go in the command block,then change equipment to your item say for example:
/summon Giant ~ ~ ~ {Equipment:[{id:diamond},{},{},{},{}],CustomName:Dinnerbone,Attributes:[{Name:generic.followRange,Base:0},{Name:generic.knockbackResistance,Base:1}],ActiveEffects:[{Id:14,Amplifier:1,Duration:999999,Ambient:1}],NoAI:1} You see I've changed the item to diamond instead of iron_sword.Good luck command engineers :)