how I can use the FFT data to compute the convolution?
You can pad zeros in frequency domain. But you need to take care about Nyquist bin when N is even.

A3 = [A2(1:N/2) A2(N/2+1)/2 zeros(1, 2*N-length(A2)-1) A2(N/2+1)/2 A2(N/2+2:end)];

Here Nyquist bin is A2(N/2+1)/2 and zeros are padded in the middle of the spectrum

For N-odd

A3 = [A2(1:(N-1)/2+1) zeros(1, 2*N-length(A2)) A2((N-1)/2+2:end)];

This is only for the case if you need to interpolate in time-domain. Almost irrelevant to the problem you have.

Posted the solution to you problem below.