If the opac is showing the incorrect Material Type Icon in the results screen, it is corrected in the bibliographic record item type.
If you are the only library attached to the bib record, correct it and save. The change will be immediate (after refreshing the page)
If there are other libraries attached, check other description fields to make sure that this bib record is for the material type that you need. If you believe it is simply an error in the mat type because all of the other physical description, correct it.
If the other physical description does not match, you need to bring in or create a new record with the correct mat type rather than attaching to this one.