Change the value of a variable in 1 script with another script?

You cant, even with a module script. Even if there was a way to do that directly, its easier to just use a value. Put which value object you want inside a script, then you can change it with other scripts. Make sure when changing Values, you dont just locate to the value's name and try to change it, you have to change the .Value property of a value object for it to work properly. Or just access the value for the script itself to work same thing, Valuename.Value.

so in this script:

local var = "k"

game.Players.LocalPlayer:GetMouse().KeyDown:Connect(function(KeyPressed)
    if KeyPressed == var then
        if script.Parent.Main.Visible == false then
            script.Parent.Main.Visible = true
        else
            script.Parent.Main.Visible = false
        end
    end
end)

Put a Value inside name anything, lets say name it "var".

ANd then to acess variable change tat value so the other script would be:

TextBox.FocusLost:Connect(function(enterPressed, inputThatCausedFocusLost)
    if enterPressed then
        script.Parent.Parent.Parent.LocalScript.var.Value = [value to set it to]
    else
        print("Player pressed", inputThatCausedFocusLost.KeyCode)
    end
end)