alarm will not disarm on key fob?

I changed the battery on my 51 plate focus last week but since then the key fob will lock doors and arm the alarm but will not disarm teh alarm even though it will unlock the doors. I have to use a manual overide switch to turn off alrm. How do I programme keyfob to disarm alarm???? I have all manuals for alarm which is a toad 850 alarm but am struggling to get this fixed.