How do i change a gui's label using a value?

This Might Work.

local n = game.Players.LocalPlayer.PlayerGui.ScreenGui.Ki
while script.Parent.Activated == true do
n.Value = (n.Value - 1)
    game.Players.LocalPlayer.PlayerGui.ScreenGui.PUTTEXTLABELNAMEHERE.Text = n.Value
    wait(1)
end

You are trying to change an IntValue's value through a LocalScript, I would suggest doing this;

local ki = 100 --change 100 to whatever amount is your default ki will be
local maxKi = 100 --change 100 to whatever to set the max ki a player will have
while script.Parent.Activated == true do
    ki = ki - 1
    if ki <= 0 then break end
    wait(1)
end

First, we are going to define ki as 100 and maxKi as 100. Ki will be your default/regular amount of ki your player will have. The maxKi just means the maximum amount of ki a player will have. Although they are just defined as int value meaning they are numbers until you used them correctly, like show above. Next while the player is clicking down with a tool, the ki will decrease by 1 every second (which you know already). If ki is less than or equal to 0 then it will stop the while loop and skip the while function.