You can replace text in references by selecting a reference and clicking Edit in the right pane.
Use the Save button to save the changes.
There are two other possibilities in EndNote to change texts in references.
In this case you work in more references simultaneously.
First there is the possibility to replace text in all references or a selection of references.
This goes via Library - Find and Replace. You will see the screen below.
Here you can indicate in which field you want to replace text (e.g. Author).
You can also choose to replace text in all fields (Any field).
Then you indicate which text must be replaced by another text.
In one action you can also change the contents of a field in all references (or a selection of references).
This can be done via Library - Change/Move/Copy Fields.
Here you can add text at the beginning or at the end of a field, replace the complete content by something new or empty a field.
See the screen below.
It can be convenient to use this option, for instance after importing a number of references.
To these "Imported references" you can add a certain feature (originated from…; or a subject that goes for the whole selection).
You can copy the contents of a field to another field via the Move/Copy Fields tab in the screen above. You can choose whether you want to keep the original content or not.