Press the Wide icon 

Press Wicket

You will be presented with the screen below

Confirm the Batter and click on the Dismissal type. Select 'Stumped'

If you have pre-selected the Wicket-keeper, their name will appear as the Fielder name. If it wasn't the Wicket-keeper that stumped the batter, then you can choose the fielder's name from the drop-down list

Press Done

Confirm the wicket details by pressing the Tick