Dell Dimension 2100 System Reference Manual page 67

2.
Remove the computer cover
3.
Rotate the power supply
4. Pry the battery out of its socket with your fingers or with a blunt,
nonconductive object such as a plastic screwdriver.
To replace the battery:
1. Install the new battery with the "+" side facing up.
2. Rotate the power supply back into place, replace the computer cover,
and reconnect your computer and devices to their electrical outlets and
turn them on.
3. Reboot the computer, press <Del> when the blue Dell logo screen
appears to enter the system setup program, and restore the correct
settings.
System Board
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