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While reading the book, tap the Plus icon to increase the speed of your audiobook, and the Minus icon to decrease the speed of your audiobook. You can see the plus and minus icon along with other control buttons like volume up, down, play, pause etc
If you don't see this Plus or Minus button, then while reading the book press the menu button, select Narration speed. Select the speed you wish, like 1X, 1.5X, 2X etc
See this video --> https://www.youtube.com/watch?v=R0N_RZ-XsnE