Change the TextureId of a hat that's being worn?

Hi, I'm new to Lua, so please try to be as specific as possible.

I'm trying to change the TextureId of a hat that's currently being worn, as the title says. This is what I've tried:

ASDF = game.Workspace.player.HoodedAssassin.Handle.Mesh

game.Players.PlayerAdded:connect(function(player)
    repeat wait()until player.Character
    wait(4)
    ASDF.TextureId = "http://www.roblox.com/asset/?id=45545728"
    end 
end)

It doesn't seem to work, and any help would be appreciated. Thank you!