Google Play Music and its associated Windows Chrome webapp suck major balls on the user interface/navigation front! I just want to free up some space on my HTC One M8 phone to do a full backup to the 32G SD card, but can't do that with 26G of music on the card. Its already stored in Itunes on my PC so no permanent loss. After much searching and trying everybody else's methods, no joy. So here you go, quick and simple.
1. Go to phone Settings/Storage
2. Scroll down to the SD Card section. It will show you different categories of files.
3. Below these, you will find Make More Space
4. Select Music.
5. Individually check songs to delete, or up in the top right corner menu, you can Select All.