STFT in Matlab

The following matlab segment, adapted from §F.3, illustrates the above processing steps:

Xtwz = zeros(N,nframes); % pre-allocate STFT output array
M = length(w);           % M = window length, N = FFT length
zp = zeros(N-M,1);       % zero padding (to be inserted)
xoff = 0;                % current offset in input signal x
for m=1:nframes
  xt = x(xoff+1:xoff+M); % extract frame of input data
  xtw = w .* xt;         % apply window to current frame
  xtwz = [xtw(Mo2+1:M); zp; xtw(1:Mo2)]; % windowed, zero padded
  Xtwz(:,m) = fft(xtwz); % STFT for frame m
  xoff = xoff + R;       % advance in-pointer by hop-size R
end

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.

I have one question please. I would like to know how to compute the Inverse STFT after having done some computations and modifications on the array given by the STFT.
Thank you

For that, you do what is called "overlap-add" reconstruction. That is, after performing the inverse FFT on each column of the Xtwz matrix, they are shifted by (m-1)*R samples (where m is the column number) and summed. See http://www.dsprelated.com/dspbooks/sasp/Overlap_Add_OLA_STFT_Processing.html
for more details.