I found a video which you might find helpful. The melodica in the video has the opposite problem – the note only sounds when blowing hard, meaning the gap between the reed (the tiny piece of thin metal), and the reed plate (the big piece of metal that are all the tiny pieces are attached to) is too big. Here they are closing the gap.
Your gap is too small, so you have to open it a bit. You need to work on the first reed – the big one nearest to the mouthpiece. Open it carefully until it looks similar to the ones next to it.
If you need to work out how to open the melodica, you need to unscrew 2 sets of screws – see how to do it in these instructions for tuning a melodica.