HP 15c - Manual (Page 63)

Section 5: The Display and Continuous Memory 63
Resetting Continuous Memory
If at any time you want to reset (entirely clear) the HP-15C Continuous
Memory:
1. Turn the calculator off.
2. Press and hold the = key, then press and hold the -
key.
3. Release the = key, then the - key. (This convention
is represented as = / -.)
When Continuous Memory is reset, Pr Error (power error) will be
displayed. Press any key to clear the display.
Note: Continuous Memory can inadvertently be interrupted and
reset if the calculator is dropped or otherwise traumatized.