In order to edit a bet you have already placed, simply hold your finger down over a bet you wish to edit. In the example below, we will update a bet that was incorrectly recorded as a 10-unit bet.
After you have held down your finger for a moment, you will see the betslip appear. From here you can choose to delete the bet completely, or simply edit the bet details:
After changing the 'Risk' value from 10 to 1 and pressing the 'Save All Edits' button, my edited bet now correctly displays on the My Action page: