add or delete a Keyword
Assign a keyword to a list of contacts. On the list, click on 
«assign or delete keywords», choose or create the keyword you want to assign to tes contacts of this list.
Don't forget to click on «enter» to create the keyword and next on « Add» to add it to all contacts on the list.
To delete a keyword on a list of contacts, choose the list of contacts with the keyword and delete the keyword.
The keyword is added on the contact sheet also:
You can also delete the keyword directly on a contact sheet