Mainly use programs such as the MAPC2MAPC converter which allows users to transform map data to various formats. It is also able to convert Google Compressed Keyhole file (.kmz) to Google Earth Keyhole file format (.kml).
Alternatively, you can just rename kmz to zip and extract the content. Then import the .kml file to Google Maps. Just be mindful of the 3 MB upload size, it won't work with larger files.