In appDesigner, you can change the value property of the button you want to have selected.
To test, I created a new app and added a Radio Button Group. This adds three radio buttons with names button, button2 and button3. The first radio button is selected by default. I also added a Button with a callback function containing the following code:
When I press the button, it sets the value of button2 to 1. The Radio Button Group will automatically deselect the first radio button and show the second as selected.