The CMOS battery in your laptop is responsible for saving the date and time as well as some other small functions on your motherboard. It may also power the system BIOS which is responsible for booting your computer. A computer that needs the date and time reset after each boot usually indicates a failed CMOS battery.
Unplug your laptop and remove the battery.
Remove the cover below the laptop screen that holds the power button. Most laptops have two or three clips that need to be disengaged for this cover to move. A flat head screwdriver will reach the clips. They will be located behind the "F1," "Insert," and "F7" keys.
Remove the screws that hold down the keyboard and move the keyboard aside. It's attached by a ribbon cable. Leave this cable plugged in.
Take off the static shield covering the motherboard by removing all of the screws holding it down. Leave the screws in their places in the shield so you will know where to put them back. They are of varying lengths and must not be interchanged.
Remove the button-sized battery from the motherboard and replace it with a new one of the same type. The battery has an imprinted part number.
Reassemble your laptop by screwing the static shield and keyboard into place and pushing the cover down until it the retaining clips snap in.
A magnetic screwdriver can be very helpful when removing small screws, which can be difficult to retrieve if they fall into the laptop case.
Most laptops use a common button battery for the CMOS. If your laptop has a different style battery when you open it up, you may need to order the replacement from the manufacturer. If in doubt, have your computer serviced by a professional to prevent damage or data loss.