Look for the <form-value-pairs> element in input-forms.xml.
Look for the <value-pairs> element for the Dublin Core element whose values you wish to change.
Add or change a <pair> element, where the <displayed-value> element contains what the submitter sees in the dropdown box, and the <stored-value> element contains what DSpace should archive as the information's label:
This set of instructions changes the existing choices when DSpace displays an option-box or dropdown menu, as for the Dublin Core "type" and "language" elements. It does not add a new submission widget/field to the user interface; to do this, see Alter submission input forms.