From any of the dropdown lists (also known as a picklist) click on the arrow in the drop down box and click on <New/Edit>. 

You can edit any of the existing choices by clicking on the Pencil & Paper (edit) icon to the right of the item. If an item is not in use, you can delete it by clicking on the Trash Can icon.  You can only delete a choice if it is not already in use. Click Close to see the edited list choices available. 

If you want to delete a choice that is in use, you will need to identify all the constituents using that choice. You could filter the Constituents Grid to identify the constituents and use the Batch Update feature in the Actions menu to select a different option for the constituents, then you can delete that choice.