If you leave the battery out for very long, all the settings in the CMOS will revert to their defaults. You should run setup and write your settings down before you take the battery out, then run setup again after you have replaced the battery to see if any of the settings have changed. Sometimes the owner's manual or user guide will provide a mainboard diagram so that you can find the battery, and some manufacturers even provide helpful disassembly hints. It isn't uncommon to find this battery hemmed in with brackets and cables, so that it isn't immediately obvious how to get the thing out without destroying something else.