Remove the keyboard. Proceed as follows:
First use a flat screwdriver to get unter the keyboard at the green line. Perhaps you have to push in the small clip
(the bottom-left red circle in the left pic) to be able to pull slightly the keyboard. Now get another screwdriver
and push in the small clip above the ESC-key marked with a red circle. The keyboard should snap out partially.
Proceed with the remaining 3 clips in the same way.
Now turn over the keyboard and locate the BIOS-battery. It is accessible through the hole marked with an arrow:
Use a screwdriver to short-circuit the BIOS battery for 3-5 seconds. The battery is hidden somewhat right under the
metal plate. But you can reach it without removing any other parts. Look:
Get with a screwdriver between the slit marked with those arrows and short the golden contact in the front with the
Replace the keyboard. Make sure it snaps into all clips. Now, the BIOS (incl. the password) should be erased.