Batman v Superman: Dawn of Justice was reportedly just given an R-rating, but the only way you'll see that version is by purchasing the Blu-ray edition.
According to Film Ratings, a website that tracks the official ratings handed out to movies by the MPAA, the Blu-ray edition of Batman v Superman scored an R-rating for its "sequences of violence." The website doesn't mention anything, however, about excessive gore, so the violence may not be as bloody or grotesque as the rating could lead one to believe.
The theatrical version of the film will still proceed with a PG-13 rating attached to it for "intense sequences of violence and action throughout, and some sensuality." Essentially, the R-rated version that will be made available can be considered Zack Snyder's director's cut.
Polygon has reached out to Warner Bros. for confirmation that this version of the movie will be made available when the Blu-ray package is released.
Batman v Superman: Dawn of Justice hits theaters March 25.