1. Select all of the citations that you want to update (To select a whole group, go to Edit > Select All ; or CTRL-A)
2. Select Library > Change/Move/Copy Fields
2. Select the field from the list that you want to update. For example, "keywords"
3. Select "Insert before field's text:"
4. Enter the text that you want to appear in the field
5. From the Insert Special menu > select Carriage return
6. Click ok.
This video demonstrates how to add your own keywords to a batch of references that have just been imported.