Reduce peaks below -20 dB using findpeaks function

Hi all, I am finding my peaks in my Array factor using 'findpeaks',i want to reduce the peaks -30 dB.

[Peaks, Locs] = findpeaks(af) ; %Find Y value of the peaks 
X_peakVal = X(Locs) ; %Find X value of the peaks 
nbPeaks=length(Peaks);%Find the number of peaks

I want to reduce my peaks with dB and replot the optimized array Factor.

Thanks in advance for your help