While simply called Dark Phoenix in it's theatrical release, the most recent film in the X-Men franchise will be retitled X-Men: Dark Phoenix for its home release.
Somewhat ironically, Dark Phoenix was originally marketed as X-Men: Dark Phoenix before its release was delayed. There's been no official confirmation on the reason for the name change, but some speculate that it might be to make the film better fit alongside the other X-Men films already on home media.
X-Men: Dark Phoenix did poorly at the box office, making only $33 million its opening weekend, so the name change could be a strategy to partially make up for those losses.
While The New Mutants faces an unclear future, X-Men: Dark Phoenix is the most recent release in 20th Century Fox's X-Men film franchise. The cinematic rights to the X-Men, and the Fantastic Four, were acquired by Disney after its merger with Fox. There has been no official word on if and when the X-Men will be joining the Marvel Cinematic Universe, though Marvel Studios President Kevin Feige has stated his excitement about having the rights to the X-Men.
Directed and written by Simon Kinberg, Dark Phoenix stars James McAvoy, Michael Fassbender, Jennifer Lawrence, Nicholas Holt, Sophie Turner, Tye Sheridan, Alexandra Shipp, Kodi Smit-McPhee, Evan Peters and Jessica Chastain. The film is set to release digitally on September 3 and will release on 4k Ultra HD, Blu-ray and DVD on September 17.