Hello and thank you for choosing just answer.
The CMOS battery is usually a button battery, sizes vary, but typically are the CR2032. This is typical for a desktop.
But, for a laptop, it can be MUCH different.
First, laptops are often soldering the CMOS battery right to the motherboard, making them very difficult to replace, but not impossible.
Secondly, you may have to remove MORE than just the back cover....the whole motherboard may have to come out.
Third, if you take on this endeavor you may VOID the warranty.
If this laptop is NOT under warranty, and you feel confident in replacing it (even if you need to use a soler gun) you will want to first FIND the battery. The best way is with the manual. Do you still have it? If not.....I can find it.