Changing the battery is pretty straightforward - just the appropriate spanners to get the terminals off and to loosen the clamp that holds the bottom of the battery in place (might differ slightly for different versions but my 1.6 petrol Altea has a soft case that goes around the battery and is easy to remove) - just make sure you don't short the battery when removing the terminals.
Alarm going off may not be the battery though - when I had that issue, it was due to the door control module.
