I did It's quite straight forward - remove the seats, grab rail, pannier supports of you have them, then the large under-seat rear section. This will expose the alarm unit which is mounted directly below the rear seat. Remove the lid and replace the batteries. No re-programming needed.
I found pictorial instructions on the net but can't remember whether here or maybe ADVrider.
Stu.
Edited because I obviously can't type fast enough! Ojay's link is the one I used
what not to do:
carefully remove topbox and place on gravel drive
put clean rag on topbox top
carefully arrange all removed fastenings, tools etc in neat order on clean rag on topbox
discover eventually that new batteries remain in topbox ......
Did mine on Sunday. You want CR123 Lithium batteries - they are normally used in cameras and I bought some of Amazon.
On my 1200GS (2004 model) the controller is directly behind the tail light so you need to remove 5-or-so torx bolts from the rear wheel inner mud shield. There is a small black box (sat nav size) that is held in with 3 screws and clipped into the wiring harness.
Open the box and there is a small flap containing the batteries. Change them and put it all back together - and remeber to put the flap back before you reattach everything
The only difference is that the beep when you arm the alarm is louder and the dwa warning has gone.