How to inverse this function IFFT?

Z = fft(Y);

P2 = abs(Z/L);

P1 = P2(1:L/2+1);

P1(2:end-1) = 2*P1(2:end-1);

subplot(3,2,3)

plot(f,P1)

title('FFT filter signal')

xlabel('f (Hz)')

ylabel('|P1(f)|')

[Rik]

Have you read the documentation for ifft?