In the Laserfiche Administration Console, you can edit the values in the list field (Document Type) and remove the ones you no longer want or need. This will only affect assigning a value for that field in documents moving forward. If an existing document was already assigned one of the values that you removed from the list, the value will remain associated with that document.

If your goal is to also "merge" some of the redundant values into a single value and then get rid of the obsolete list value, then what I would recommend is to pick the list value you want to keep. Then do a search for documents that may be assigned one of the redundant Document Type values (i.e. using the Laserfiche Client) and then select all those documents, edit the metadata (fields), and change the Document Type to the value you plan on keeping. Then you can edit the Document Type list in the Administration Console and remove the redundant value.