how change your face to a decal with a tool?

A function in a function would be appropriate for this, I prefer it this way (Make sure it's a LocalScript since you are getting MouseClicks from an individual Player).

local Tool = script.Parent --make sure this is a Tool object

Tool.Equipped:connect(function(Mouse) --Tool equipped function
    Mouse.Button1Down:connect(function() --Button1Down is an Event for whenever the right mouse is clicked
    local Player = game:GetService("Players").LocalPlayer.Character --Character is a part of LocalPlayer, it is the player in the server where the player has shirts and pants etc.
    Player.Head.Face.Texture = 1296295817 --texture id, make sure Player.Head.Face.Texture is correct or the code will fail
    end)
end)

If it works please accept this as an answer :) I did not check the last part (Player.Head.Face.Texture) so make sure it is correct. To make sure it is correct press Play Solo, go to Workspace and find your character with your username (it will be a model), Click on head and see if there is Face.Texture inside it.