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If your drop down list does not include a value that you would like to have available for selection. Lets take an example of adding a new Notification type. Go to:
My Settings > Definition Fields
Find the menu that the drop down list belongs in, eg . Feedback
Click on the Field Name, eg Notification Type.
The current values are listed. Simply type in the new value and press the 'Add' button.
To delete, click checkbox next to the value you wish to remove then click the Delete button.
Note that if this value is being used on a record, you may not be able to delete as it will ruin historical records (data integrity).