Just hold down the button that changes the information displayed. It looks as if the 12/24 clock is assigned based on the unit of measurement you choose (Km or miles). With kilometer measurements using the 24 hour clock and mile measurements using the 12 hour clock.

I have mind held on by the ties as well (had to go buy some larger ones since my down tube is a bit oversized) but I wrapped the wiring where the ties are in electrical tape to prevent it from cutting into the soft rubber.

As far as I gather from their site, the warranty thing is on the manual near the bottom.