how to replace real time battery
your Cmos battery?
turn pc off
use cocktail stick to activate little retention clip on your battery housing
catch battery as it falls out
take to shop,get replacement 2032 cell battery
dispose of old one responsibly
go home, install new battery making sure it clips in securely
boot as normal,
repeat every 3 years or so
You may need additional steps because doing that ^ may wipe out the custom settings in your BIOS (just like doing a deliberate BIOS Reset does), depending on how long the battery is out. On the other hand, if you buy the replacement CR2032 battery first, you can insert the new one within a few seconds of removing the old one, rather than many hours or days later. The BIOS will retain its settings in its dedicated CMOS memory for a while with no battery power, but certainly not for hours.
If your battery is out for a long time it could reset your BIOS. In preparation for that, if that is your plan, you should go through the BIOS Setup screens and note any custom settings you have made, like SATA Port Modes, Boor Priority Sequence, CPU voltage and speed, RAM voltage and timing parameters, etc. Then AFTER the new battery is in, check through again to see if anything was changed. If not, you're done. Even if the reset happened because of a long time with no power, the BIOS' CMOS memory will not be empty - it will automatically restore a "cold boot" default set of parameters. However, they may not be the ones you had before.
If you do detect changes it indicates the BIOS was at least partly reset. In that case, the best path is to completely reload the BIOS settings and then re-do your own customizations. To reload a reliable set of BIOS parameters, look near the end of the BIOS pages for a place to load defined settings. Load with the Factory Default or Optimal set, Save and Exit. As it reboots, go back into BIOS Setup again and re-set all the custom settings you noted. Save and Exit, and now you're done.
I understood motopsychojdn's urging to responsibility, too. Letting an old battery rot until the case bursts and the metal oxidizes into green paste goo does not qualify as "green disposal"! That is one small consequence of the sequence I proposed. You would have to make a second trip back to the shop sometime to leave the old battery in their disposal bin.