According to the manual you have five minutes to replace the battery after removing the original or you will lose all settings. The battery is under the memory cover on the bottom of the laptop. These are the instructions from the manual. You can find it at : http://h10032.www1.hp.com/ctg/Manual/c01951704.pdf
Before removing the RTC battery, follow these steps:
1. Shut down the computer. If you are unsure whether the computer is off or in Hibernation, turn the
computer on, and then shut it down through the operating system.
2. Disconnect all external devices connected to the computer.
3. Disconnect the power from the computer by first unplugging the power cord from the AC outlet and
then unplugging the AC adapter from the computer.
4. Remove the battery (see Battery on page 47).
5. Remove the memory/WLAN module compartment cover (see Memory module on page 54).
Remove the RTC battery:
Remove the RTC battery from the socket on the system board.