How do you convert midi file to wav file?

Unfortunately, this is not possible since MIDI files are not music files at all. They are instructions to the midi synth in your computer to play certain tones. Because music files (.WAV, .MP3) were created manually, they may have tones (like vocals) that can't be synthesized. Also, even if the tones could be synthesized, like if your file was just a keyboard track, software won't know how to tell the midi synthesizer which tones you want it to reproduce. Your best bet would be to play it again, creating is as a MIDI file. You need special equipment to do this, but that's another question.

You could try looking for an online file converter, but converting in this direction does not exist.

Going the other way is possible, since that would take the instructions from the .MID file and instrument samples from a sample library and essentially "play" the file into another format such as .WAV.