to mon

local sText = "EXIT" -- You can change the text
local sSide = "back" -- you can change the side of the monitor
local nTextSize = 3 -- change the text size here (from 1 to 5)
local nTextColor = green

local function printCenter(mon, txt)
  local w, h = mon.getSize()
  local x = math.floor((w / 2) - (#txt / 2))
  local y = math.floor((h / 2))
  mon.setCursorPos(x, y)
  mon.write(txt)
end

local monitor = peripheral.wrap(sSide)
printCenter(monitor, sText)