cmos battery 6 years old

If you make the switch really quickly, there may not be a need to do anything. However, it's always best to check.

Before you replace the battery, go into your BIOS and write down all settings. After you replace the battery, the BIOS may have been reset to factory default. At the very minimum, the system date/time will be wrong, so set it. Then, look to make sure that everything else is set exactly the way it was before the replacement.

To find a professional store purchasing the battery, and ask them to teach you. I've ever done it, but I forgot. My purchase was placed here: http://www.laptoptraveller.com/laptop-battery-computer-notebook-batteries.html

They are a patient store that replying me for 4 times to answer my questions. You may consider asking them.