Please note that the previous reply implied that pressing the [C ALL] key (down-shift C) is the answer to your question. It isn't. You asked for the way to "clear" the P/YR setting, but the [C ALL] key only displays the current P/YR value without changing it.
You can change the P/YR value to anything you wish by keying that number and then pressing the [P/YR] key (down-shift PMT). For example, to set monthly payments, key 12 then press [P/YR]. WARNING: do NOT press down-shift N. That's the ×P/YR key, which is an entirely different function. (See the manual for details).
To clear the entire machine, which includes resetting P/YR to 12, perform the three-finger Master Reset (press ON and N and FV all at the same time, then release them at the same time). You'll see an HP Copyright notice, followed by ALL CLEAr. Pressing [C ALL] or [RCL] [down-shift] [P/YR] will then show the current P/YR value of 12.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.