In the iTunes 12.4 update that Apple released this week, a number of features were removed, in part to simplify the contextual menus. One of the was the Reset Plays feature, which you could use to set the play count for selected items to 0. Many people used this so they could have items show up in smart playlists, where a condition was Plays Is 0.
Doug Adams has posted an article discussing this, and linking to three AppleScripts he has available on his site: Add or Subtract Play Count, New Last Played Date, or New Play Count. Each of these scripts can change play counts, but Doug also offers a script that can reset play counts completely. (It’s a roll-your-own script; he provides the code, and you open it in Script Editor, then save it.)
So if you’re missing this feature, check out Doug’s article.