Tate was almost nine months pregnant when she was murdered along with four friends at the home she shared with husband Roman Polanski, a director.
Tate's sister initially criticised the film but has now given it her support.
"First look. #OnceUponATimeInHollywood", Margot captioned a photo in which she sports her blonder-than-usual hair, a black top, a white skirt and a pair of white boots.
Leonardo DiCaprio will play struggling actor Rick Dalton, who starred in a Western TV series but failed to build a successful film career.
However, she has since discussed the film with Tarantino and his team, and given it her full support.
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Tarantino's movie, which features Leonardo DiCaprio as Rick, an actor, and Brad Pitt as his stunt double, Cliff, was originally slated to come out on August 9, 2019.
The film was originally slated for release in the United States of America on the 50th anniversary of the LaBianca murders, one day after Tate was killed, but has since been moved to July 26, 2019. I'm very excited to tell this story of an LA, and a Hollywood that doesn't exist anymore. Tate reportedly appears as a neighbour.
Once Upon a Time in Hollywood is slated to arrive in theaters on July 26, 2019.
Debra had previously condemned films being made for the 50th anniversary, including Once Upon A Time In Hollywood and The Haunting of Sharon Tate.