At the time of writing (App version being current) it is not possible to change the Mute buttons behaviour. 

This means that the individual mute buttons below the faders work as a toggle (on/off) switch and the mute in the bottom right is a Hold switch, and when you release it unmutes. 

The idea behind this is so that the Mute in the bottom right can be used if you need to cough/sneeze or have a drink and quickly mute yourself. 

This has been reported to the development team as a feature request and we hope to see it in the near future.