Go into iTunes, find the album and select it, right-click for INFO and hopefully the Release Date info is shown, then u can change right there.
If INFO doesn't show then u have to manually edit the metadata info on the files (mp3?) themselves, ASSUMING they are on ur HDD and not Music-Matched (coming from Internet). U have to use an appropriate editing tool because there are several versions of ID3 metadata. THEN once that's done u have to tell iTunes to refresh the new data.
If by now this all seems too much work for a little annoyance....