The Mixer reshapes iReal Pro's backing band. Open it from the Player controls: on iOS and Android tap the chart, then tap the Mixer button (second from the right); on Mac and Windows show the Player controls panel from the View menu or the icon at the top right.

 

Tap or click an instrument's name to change its sound, use the sliders for volume and reverb, and slide a volume to the far left to mute a part.

 

For the full guide, see Customize your backing tracks on the iReal Pro website.