There is no app for the Ui series as everything is built into the mixer. It works as a WiFi hotspot you connect to and control with the built-in website.
Refer to the 3 step connect guide for your model:
https://help.harmanpro.com/how-to-connect-and-run-a-ui12-mixer-in-three-easy-steps
https://help.harmanpro.com/how-to-connect-and-run-a-ui16-mixer-in-three-easy-steps
https://help.harmanpro.com/how-to-connect-and-run-a-ui24-mixer-in-three-easy-steps
