I found a CMOS restart switch then I followed the instructions on page 91 of the Lenovo ThinkCentre M93/p Tiny User Guide:
This section provides instructions on how to erase lost or forgotten passwords, such as a user password.
To erase a lost or forgotten password, do the following:
- Remove all media from the drives and turn off all attached devices and the computer. Then, disconnect all power cords from electrical outlets and disconnect all cables that are connected to the computer.
- Remove the computer cover. See “Removing the computer cover” on page 53.
- Locate the Clear CMOS /Recovery jumper on the system board. See “Locating parts on the system board” on page 13.
- Move the jumper from the standard position (pin 1 and pin 2) to the maintenance position (pin 2 and pin 3).
- Reinstall the computer cover and connect the power cord. See “Completing the parts replacement” on page 78.
- Turn on the computer and leave it on for approximately 10 seconds. When you hear short beeps, turn off the computer by holding the power switch for approximately five seconds.
- Repeat step 1 through step 2.
- Move the Clear CMOS /Recovery jumper back to the standard position (pin 1 and pin 2).
- Reinstall the computer cover and connect the power cord. See “Completing the parts replacement” on page 78.
Figure 4 “System board part locations” appears on page 13:
The Clear CMOS (Complementary Metal Oxide Semiconductor) /Recovery jumper is item 6.