If you use a Umo Card instead of the Umo App, you can add stored value to your account at the Umo Passenger Portal by going to My Wallet > Buy Value or a Pass > Stored Value > Choose Amount.
If you use the Umo App, you can add a cash balance ("stored value") by going to Wallet > Cash Balance > Add Cash. (See full instructions here).
For instructions on purchasing stored value at the Umo Passenger Portal, see Buy Stored Value.