You need two new CR-123a batteries. Before you start installing batteries make sure that you have disarmed the 3-Series Security Keypad 3400, as it might alert alarms while you are working on it. Follow these steps to install batteries
If your keypad is mounted on a wall, then remove it from the mount easily by sliding it off the mount.
Then remove the battery cover found behind the keypad by pushing the back cover.
Remove the old batteries.
Now before you place the new batteries look at the diagram given in the compartment and place the batteries accordingly.
Then replace the back cover to its original position by pushing it up.
You know that you have inserted the batteries right by the indication of green light in the keypad.
Finally, you can place the keypad back in its mounting stand.