Just follow these steps:
1. Go to Settings -> General -> Storage & iCloud Usage.
2. Tap Manage Storage and wait till you get a list of your iPhone apps. …
3. Choose Music, then click Edit at the top right corner and All Songs in case you really want to get rid of all your music.
4. And finally tap the Delete button.
Learn how to delete songs, albums, or all music from your iPhone, iPad, or iPod touch. This quick and easy tutorial will help you remove unwanted music from any iOS device. To delete a single song from your iPhone, tap the Music icon on your home screen to open the app. Tap Songs to see your entire music library. Find a song you want to delete, then tap and hold on the song’s name to open a pop-up menu. To delete the song, tap Delete from Library. To confirm, tap Delete Song. You can also delete entire albums from your device by tapping Library at the bottom, then selecting Albums. Find an album you want to delete. Tap and hold on the album cover to access the pop-up menu, then tap Delete from Library. To confirm, tap Delete Album. Another way to manage your iPhone’s music library is through your device’s settings. Open the settings app on your iPhone, then select General. Tap iPhone Storage. Scroll down and tap Music. To remove all songs in your music library, swipe your finger to the left over All Songs, then tap Delete. If you want to delete all songs by a specific artist, swipe your finger to the left over the artist’s name, then tap Delete. Tap an artist’s name to see the albums and songs you have on your iPhone, you can also swipe left to access the option to delete any album or song if you’d like.