How do you kill the Wither in Minecraft PE using commands?
/kill @e[type=Wither] should do the trick. The command finds any entity that has the type "Wither" and then kills it.
Warning that the wither needs to be loaded in for this to work. You'd know it's loaded in when you see a "Wither" health bar at the top.
This should work on PE/Bedrock and Java edition.