A field is a rather static object. Fields usually don't change their position or their geometry. Yet it still might be usefull to change some details.
To edit the administrative data of a field just right click the field and select 'open'. Change the data in the newly opened window, and hit save. Easy, isn't it?
Editting the geometry isn't possible yet. The best thing to do when a geometry has changed is this:
By doing this you make sure that the history of fields stays correct, which is a good thing.