HP DeskPro 405 G6 DM ( https://support.hp.com/hr-en/document/c06955645 ). In this case 23H62EA with 872E board AM4 socket.
Here are images how to disassemble the computer, it may help someone.
First,
Regarding RAM upgrades, check this thread.
Regarding BIOS update: BIOS (F1 key), BIOS main...