First, navigate to your presets list, under the My Presets section.
iOS:
Android:
Then, select the preset you'd like to use.
When this preset detail is active, the amp should update in real time to reflect your tone selection.
Tip: You can also swipe up and down to cycle through your presets without having to return to the list, and swipe left from the preset detail view to enter the signal chain and navigate through it by swiping.