General Motors Corp. announced top-level management changes yesterday, the same day the Detroit automaker reported an 11.5 percent decline in net 1985 profits from year-earlier levels.

The changes came as GM was reporting its profit margin was down significantly in 1985 compared with 1984, to 4.1 percent from 5.4 percent.

Despite lower earnings for the year, GM said sales were up 14 percent to $96.4 billion from $83.9 billion, allowing it to regain the ranking of world's largest corporation, unseating Exxon Corp., which reported $93.2 billion in revenue.

Earnings per share of GM common stock were $12.28 compared with $14.27 a year earlier, which included a $1.34-per-share tax credit.

Sales in the fourth quarter were $24.6 billion compared with $20.9 billion during the same quarter a year earlier, GM said. GM said fourth-quarter earnings were more than $1.2 billion ($3.85 a share) compared with $877 million ($2.76) a year ago.

Included in the management changes:

*Alexander A. Cunningham, executive vice president in charge of North American Automotive Operations, was placed on an immediate disability leave of absence followed by retirement, GM Chairman Roger B. Smith and President F. James McDonald said in a statement.

*Lloyd E. Reuss, formerly vice president and group executive in charge of the Chevrolet-Pontiac-GM of Canada Group, succeeded Cunningham in both the operations vice presidency and on the Board of Directors, the statement said.

Robert C. Stempel, former vice president in charge of the Buick-Oldsmobile-Cadillac Group, was promoted to executive vice president in charge of both the truck and bus and overseas groups, and was elected to the board, it said.