I finally got this to work. Unlike what the description here says, this is how to update the value:
In Home Assistant, go to Developer->Service->YAML mode:
service: text.set_value data: value: "967067" target: entity_id: text.water_meter_100_2
(change 967067 with the value you want to add to the current counter)
The service "notify.mysensors" seems to be deprecated by Home Assistant.