HP 40gs and 39gs Factory Reset
Here is the procedure for Factory or hard (full) reset the HP 39g+, HP39gs and HP40gs. Warning, DO NOT do this unless you are sure you don’t need anything stored on the calculator! If you do it then you will find that the calculator’s memory will be completely wiped, resetting it back to the default values it had when you first opened the box.
You need to press the following 3 keys at one time - Press and hold the first and last (sixth) screen keys on the top row, then press the ON key. The calculator will reboot and the screen will display ‘Memory Clear’. Press Enter or the ON key.