In the library window, double-click on one of the references that you have entered. This will open it for editing.
You can change any of the details. For example, add some text to the Notes field. Close the reference when you have finished editing and this will save the changes.
Alternatively, to make a small change quickly, you can click on the reference once to highlight it, and then click on the Reference tab in the right pane of the Library window.
Once you have finished making your changes, go to File and select Save (or you can click onto another reference- EndNote will ask if you wish to save your changes- select yes).