You can set engine hours to desired value inside web application - https://web.sentinelmarine.net/.
1) Login into web application using same credentials as for mobile application
2) Select desired boat and/or device
3) In the bottom panel search for Engine hours
4) Click on Engine hours sensor
5) On the right side of the screen, new panel will open with sensor properties
6) Click on Change text
7) Input desired value
8) Confirm by clicking Save