The United Auto Workers union yesterday reversed itself for the second time in a week, as leaders voted to go back to the bargaining table with General Motors over wage and benefit "givebacks" that the company has promised would be reflected in lower sticker prices on cars and trucks.

The union's present contract with GM expires Sept. 14. But UAW leaders agreed last month to begin bargaining early and to discuss givebacks instead of the usual increases in compensation in hopes of helping to shore up GM, whose sales have fallen precipitously along with those of other U.S. automakers.

The union, which has been losing jobs and members as the companies lost sales, has been in similar negotiations with Ford Motor Co.

Last week, however, it broke off the leading talks, those with GM, in disputes over the size of wage and benefit concessions, job security for remaining members and how long the company price concessions would endure.

But yesterday, the divided UAW-GM council refreshed the mandate of the bargaining committee to discuss concessions and sent it back to the table. The new "final deadline" for reaching a tentative agreement is midnight Thursday.

The Auto Workers are one of several large unions discussing givebacks in this year of recession and dwindling jobs. The Teamsters also made concessions in a contract tentatively agreed to earlier this month.

If GM fails to meet the new bargaining deadline it will have to wait until regularly scheduled bargaining in July to try to win the labor cost savings it says it needs to stem business losses and become more competitive with foreign car manufacturers, particularly the Japanese, according to UAW President Douglas A. Fraser.

"This is the final chance, the final, final chance. If you don't do it now, you can forget about it until July," Fraser said.

GM officials repeatedly have said they cannot wait that long for a new agreement. They have threatened to send more work overseas, where it can be done at lower cost, and to close some domestic car manufacturing plants if no early settlement is reached.

The union and the company had hoped to have a tentative settlement yesterday to present to the union's 300-member GM council, which met here yesterday in conjunction with the opening of the UAW's seventh annual community action program conference. But the talks, which began Jan. 10, became deadlocked Jan. 20.

UAW emergency talks with Ford broke off at the same time.

The GM council, casting weighted ballots in a roll-call vote demanded by members, voted 2,277 to 1,694 (57 percent to 43 percent) to resume talks Tuesday in Detroit. UAW members waiting outside the meeting room in the Washington Hilton yesterday booed and shouted "sell-out" when the vote was announced.

UAW representatives of Ford workers received a similar greeting when they announced they would return to the table Friday, one day after the union's deadline for a new agreement with GM. There was no roll-call vote in the 225-member UAW-Ford council meeting.

Fraser acknowledged the growing rank-and-file discontent. "I'd rather have a bigger margin" of approval, he told reporters after the meetings. But he said the discontent should increase pressure on GM to come up with "a well-balanced agreement that could be taken to the rank-and-file members for ratification."

Such an agreement, from the union's viewpoint, would be a short-term contract expiring "somewhere in the neighborhood of September, 1983," Fraser said. It would contain labor cost savings that could produce car price reductions amounting to more than a few hundred dollars per unit, although Fraser refused to specify an amount.

GM officials prefer a contract longer than the traditional three-year agreements. That would ensure long-term corporate benefits from any wage and benefit concessions.