Need to reset BIOS

I need to reset the BIOS settings on a device like this. I looked for a baterry for the cmos chip on the board to no avail. I have researched it a bit and found that the cmos chip is a windbond 25q64fvsig and that it could be reset. I looked into methods of resetting EEPROMS but I'm just not sure how to go about it. I have the datasheet on the chip, ut I'm useless with electrical engineering.

The reason I need to reset it is to work on the OS repair I meant to do. I changed the BIOS settings for the machine to boot to legacy and now I can't get to BIOS, as the machine powers on and the VAIO screeen shows, I press key combinations in attempts to reach BIOS without success. After the logo a screen displaying "operating system not found" and as I pres the key combinations it re-itarates this message. I can boot into USB drives but since the machine has windows it and a GPT partition it doesn't allow me to edit the SSD.

Before all this the user said that the machine had gotten wet, yet there are no sings of water damage other than a bit of oxydation on the copper radioator from the heat sink. With a Windows Console inside a Windows 10 installation from a USB drive I was able to read files from the SSD so I believe it to still be alive.