If signal(1D) is buffer and buffer matrix(2D) is process after this how to combine back the processed buffer matrix to generate the same length (1D) signal as input?

If I have the signal

A=[1:100]; % 1D signal

B=buffer(A,20,10); % framesize=20 and frameshift=10. % B is 2D Matrix.

Now need to find FFT of each column and want to combine back the result to get the output 1D of the same length as input.

i tried with following code

B_FFT=[];

[m,n]=size(B);

for i=1:length(A)

    B_FFT=fft(B(:,i),Fs);

    F1=abs(F(1:Fs/2));

    h=fir1(25,17,'low');

    y=filter(h,1,A);

end

Now need to combine output y to form the same length as input.