GarageBand app does not support export of WAV natively, so your options for exporting .band files (music projects) are somewhat limited. It was reported to us that you can use Apple Logic Pro app that supposedly is able to import .band files and export audio files as .wav files. Whenever this is actually possible could not be reliably verified.
Convert GarageBand music project to WAVe PCM Sound, standard Windows sound format.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert band to wav on OS X
A digital audio workstation and MIDI sequencer program
Convert from band file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.