To modify an existing observation, go to Menu/Observations, locate the observation and click on the observation. You will see an entry screen that is similar to that of a new observation, with all observation details filled in. Note that instead of “Add”, this screen now has “Update” as title. The behavior is a little different from adding a new observation:
Note that you can only modify observations that are not yet uploaded. As soon as an observation has been uploaded, it cannot be changed in iObs anymore. To modify observations that have been uploaded, go to one of the portals, log in, locate your observations and make corrections as you see fit. Changes to observations made on the portal are not copied back to iObs.