Lester Bernstein was named editor of Newsweek magazine today replacing Edward Kosner.

Katherine Graham, chairman of the board of the Washington Post Co. which owns Newsweek, announced the change which brings back a former Newsweek managing editor who left the magazine six years ago to become vice president for corporate communications at RCA.

Bernstein, 58, will rejoin Newsweek Aug 15. Kenneth Auchincloss, the magazine's managing editor, will serve as editor until then. Bernstein started at Newsweek in 1963 after having worked at the New York Times, Time Magazine and NBC. He was national affairs editor and executive editor before becoming managing editor during his 10 years with Newsweek.

In a memorandum to the staff, Bernstein said there inevitably will be some changes in tone and operating style that evolve during his editorship, but that he has no intention of trying to reinvent the news magazine's style "which is sturdier than all of us."

"In 10 years at Newsweek," Graham said, "Lester was one of a team of top editors who broke old molds and set new standards for the newsweekly form. He brings professionalism, intelligence, wit and warmth to every job he undertakes.I know that he and the Newsweek staff enjoy a professional and personal rapport that will keep the magazine advancing in the years ahead."

Bernstein left Newsweek when Osborn Elliott returned for a second stint as editor of the magazine in a move that appeared at that time to block Bernstein's chances of winning Newsweek's top job.

The sudden firing of Kosner, 41, who was informed of the decision Tuesday, apparently resulted from dissatisfaction with a number of developments.

Newsweek recently introduced a new design which has met with considerable criticism inside and outside the magazine.

A number of staffers have been critical also of a trend toward stressing the more frivolous, show-business elements at the expense of hard news.

For example, Newsweek ran a feature story on the latest wave of horror movies on its cover June 18 rather than the Pope's triumphal visit to Poland.

With the end of Watergate and the Vietnam war, Newsweek gave increasingly large amounts of space to celebrity and entertainment stories, perhaps feeling new competitive pressure from the success of People Magazine.

There were also complaints about Kosner's managerial style and morale has not been high at the magazine recently.

Kosner was editor for four years and worked at Newsweek for 16 years after joining the magazine as a writer. He held all the major editorial positions at Newsweek during his career and, as managing editor, directed its coverage of Watergate.

Kosner called the staff to a meeting this morning in which he said a brief goodbye and praised the employes. "I think we can be proud of it," Kosner said of Newsweek in recent years.

He left the Newsweek building immediately after the staff meeting at which Graham announced the choice of Bernstein to succeed him. Kosner's future plans were not known.

Newsweek has a circulation of more than 2.9 million and in 1978 carried 3,323 pages of advertising.