How do i make a gui that will change when you touch a certain part in a game?

You need to use a Touched event for when the character touches the part, and then you need to get the player from the character, then you need to modify the GUI in the PlayerGui. Put this in the part:

debounce = false
script.Parent.Touched:connect(function(hit)
    if debounce == true then return end -- Make sure it doesn't modify more than once
    debounce = true
    if hit.Parent:findFirstChild("Humanoid") then -- Make sure it's a player touching it
        player = game.Players:getPlayerFromCharacter(hit.Parent) --Finds the character
        if player ~= nil then --Just a safe-guard
            Gui = player.PlayerGui.MODIFYTHEGUISTUFFHERE -- Yeah
        end
    end
    debounce = false
end)

You need to learn the touched event. Then somehow find the Character. After that do :GetPlayerFromCharacter() THen go to the the PlayerGui and ScreenGui And TextLabel. Then change the text.