To edit your observation, go to your user menu and click on "My Profile".
All the observations you uploaded to WhatSpecies are displayed at the bottom of the page in a similar manner to the WhatSpecies homepage.
Click on the observation you want to edit and then click on the "Edit Observation" button in the top left of the page.
After you have completed editing your observation, click on "Save".
There, your observation has been updated.