Modern Vespa

Any easy way to change it out with a new one? The wiki on the GT200 has broken pictures.

Sorry the picture links are broken. There is a hyperlink to the headset removal images which is the biggest part of the job.....
GT/GTS: Headset Cover Removal

Once you have the dashboard loose you can see the small clear plastic battery cover deep in the back of the instrument panel.
WARNING: Either completely remove the dash and do this job at a table or have a 'catch-all' sheet over your the top of the steering column as there is a tiny spring washer inside the battery housing that just loves to fall out...either on the way out, or on the way in. It tends to disappear into the chassis never to be seen again
The little battery cover is a simple 1/4 turn to unlock/lock.
The battery will be available at your local automotive store/watch repairer/supermarket...pretty common size.

Good luck.

The spring is after I open the battery cover a quarter turn? Is it possible to use a tamper set tool and to release and lift the gauges up (on the s150 model) instead of taking the head set apart?

I changed the clock battery on my GT200 a couple of years ago. You have to access the rear of the instrument panel. The best way is to remove the headset and do it on a workbench. I tried to find the youtube video for you but it has been removed. Boulty has you headed in the right direction.

I'll add be careful with those little tabs when taking the headset on and off. They break VERY easily. Good luck