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You can shuffle your entire song library directly in the Music app. This will display all of the songs in your library. You'll find this at the very top of your list of songs. If you already have a song playing in Music, you can shuffle from the Now Playing menu:  Tap the mini player at the bottom of the Music app screen to expand it. Swipe to scroll down on the Now Playing screen. Tap the Shuffle button next to "Up Next." This will shuffle all of the songs in your library. Siri can shuffle your music from any screen:  Press and hold the Home button to start Siri. Say "Shuffle songs." Siri will shuffle all of the music in your library. The Control Center on your iPhone allows you to start your music from most screens:  Swipe up from the bottom of the screen to open the Control Center. Swipe from right to left on the Control Center to open the Music controls. Press "Play" to start shuffling your music. If your last-played song starts instead, double-tap Home and clear Music from the list of recent apps. This will clear it out of the Control Center.
Open the Music app. Tap the "Library" tab and then tap "Songs." Tap the "Shuffle All" button. Shuffle from the Now Playing menu. Use Siri to shuffle your music. Use the Control Center to shuffle your music.