how can ifix my [watch,casio g-shock,g-2900]?

You need to reset the module after you change the battery. Remove the back of the watch. At the top of the battery you should see AC marked on the battery cover. Take a metal tweezers, put one prong on the back of the battery and the other prong on top of AC. Hold for a few seconds. Turn the watch over, if the display is showing the module is reset. Replace the watch back. You will have to set the time, date, and reenter any e data information that was stored in the watch.

Dan Schopp
June 2008