mIRC don't have such alias to make it directly, but you can do it fast ussing send keys(you must say thanks to qwerty for his sendkeys alias)..

Example of ussing: /ChangeLines 3
Script will only work when mIRC will be the active application.

Code:
Alias ChangeLines {
  if ($appactive) sendkeys % $+ o {END}{UP}{UP}{UP} % $+ l {HOME} $str({DOWN},$calc($1 - 1)) {ENTER}
}

alias sendkeys {
  var %a = sendkeys $+ $ticks
  .comopen %a WScript.Shell
  if !$comerr {
    var %b = $com(%a,SendKeys,3,bstr,$1-)
    .comclose %a
    return %b
  }
  return 0
}

mIRC Chm Help 6.16.0.3 Full Anchored!