Water usage statistics are updated on the Monitor card screen and the Monitor detail screen at every app refresh. You can perform a refresh at any time by swiping down anywhere in the app. If you experience issues with the refresh feature, you may exit the app completely and return to the main screen, at which point the usage should be updated. Assuming your Monitor is online, calibrated, and water is currently running, you will notice the water usage statistics have updated on both the Monitor Card screen and the Monitor Detail screen. If you experience issues with this, please note the app version and make sure there are no newer versions available in the store.
For more information about refreshing your StreamLabs app, please read here.