Either turn on Optimize Storage via settings/music on your iPhone, or go into your music library on your iPhone and select songs, playlists, or albums and remove the download. Remove download keeps the item in your library but removes the actual file from your device. Be careful about deleting from library as that removes the song from your library completely.
Delete music in the Apple Music app - Apple Support
If you want to free up storage space on your device, learn how to remove music that you downloaded from the Apple Music catalog or purchased from the iTunes Store.support.apple.com
If you are an Apple Music or iTunes Matching subscriber, go into music on your iPhone and select the album. Then, at the top portion of your iPhone screen, touch the 3 dots in the red circle. Then tap remove, then select remove download. If you are not a subscriber of these services and are just syncing with your computer, then this option may not be available.