Sign in to app.speechify.com using your Speechify account credentials, preferably using a Chromium browser.

Open the file that you want to listen to.

Click on the 'Speed' option with the face on the left side toolbar. This button also displays the speed that is currently set for your program.
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You can use the slider or the '+' and '-' buttons above and below the slider to increase or decrease the listening speed.
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If you want the app to increase your speed automatically, as per your listening activity, enable the 'Increase Speed Automatically' feature at the bottom by clicking on the checkbox. This would let the app help you increase your productivity as you listen more.