Turn the power switch off and open the battery chamber cover by sliding the battery chamber lock release lever toward the back of the camera (away from the lens).  You will need two CR2-type 3V lithium batteries.  The first battery will be placed in the chamber (-) pole first.  The second battery will be put in the same way.  Firmly close the battery chamber cover.  You will hear it lock into place.  Always replace the batteries at the same time.