You can change your weekly amount to be saved by pressing the large plus button in the home screen, and changing the amount with the '+' and '-' buttons. Alternatively you can change the amount by tapping on the amount and typing a new value. Make sure you push the tick icon to confirm this change!
If done correctly, the change should be reflected in the "Next:" section on the home screen, and in the transactions screen.
Please note this only changes the amount to be saved on the current week. The following week will reset back to the amount you have set for your automatic weekly deposit.