Unfortunately, BASS doesn't currently include support for tag writing, only reading. So you would need to use a third-party tagging library for that, or do it yourself if you're familiar with the RIFF/WAVE file format in this case. If your changes don't affect the size of the existing "cart" chunk then it should be fairly straightforward to write them to the file (if the size is changed then you may need to shift other chunks too depending on where the "cart" chunk is located).