How do I make a gui button that everytime you click on it it changes to another word?

Here you go :

script.Parent.Text = "Mute"
script.Parent.MouseButton1Click:Connect(function()
    if script.Parent.Text == "Mute" then
        script.Parent.Text = "Unmute"
    elseif script.Parent.Text == "Unmute" then
        script.Parent.Text = "Mute"
    end
end)

If this helps accept the answer! Let me know if you find any problem or if it's not what you wanted.

Using a TextButton's MouseButton1Down event and its Text property

local Button = (button here);
Button.MouseButton1Down:Connect(function()
    local isText = (Button.Text == "Mute"); -- Is the text "mute?"
    Button.Text = (isText and "Unmute" or "Mute"); -- If yes then change it to "Unmute", if its not then make it "Mute".
end)