Upsampling and Downsampling using IFFT and FFT (DFT Based Resampling)
Given a time domain signal $x_1[n]$ of length $N$ and its $N$-point DFT $X_1[k]$, If you pad $M$ zeros to the end of $x_1[n]$ and then take ($N+M$) point-DFT of it, this will give you an interpolated set of frequency samples, $X_2[k]$; i.e., a denser set of uniform samples of the DTFT, $X(e^{j\omega})$, than was previously available from $X_1[k]$.
Given a time domain signal $x_1[n]$ of length $N$ and its $N$-point DFT $X_1[k]$, If you carefully stuff (insert) $M$ zeros to the central portion of the DFT $X_1[k]$ (by moving those right half of the available $X_1[k]$ samples to the right by the amount of $M$) and then take ($N+M$) point-inverse DFT of the resulting $X_2[k]$, this will give you an interpolated set of time samples, $x_2[n]$; Note that you should scale the amplitudes in accordance with the interpoaltion amount.


Note that implementing 1-) is straightforward, while implementing 2-) requires careful selection of the correct indices at the central portion for the original length $N$ being even and odd, the details of which is provided by some examples here dsp.se,  including the link you have provided.