Granted I’m new to the SQ Mix Pad app but unlike the QU Mix Pad app I don’t think there is a way to globally change and aux/mix from pre or post using the SQ app.
You can change it on a channel by channel basis by either sliding the fader control panel all the way up to show you the on/off mix select and the pre/post button at the top of fader or with the with the channel fader window slid down it will show all the mixes and groups and their selections in the top window.
To do either of those you have to have a mix/aux selected.