I'm getting the error, "object expected got table"
contestants = (game.Players:GetPlayers())
beast = contestants[math.random(1,#contestants)]
local newtorso = game.Workspace.newtorso
local torso = beast:GetChildren("UpperTorso")
newtorso.Parent = torso
end

2 comments
should be beast["UpperTorso"]
and i dont think you can replace, but you can look into welding parts/models to the player?