Reagan administration officials will try again this year to modify the two most attractive and costly features of the federal pension plan: Retirement at age 55 and full indexing to the cost of living.

The president's budget asks Congress to raise the federal retirement age to 62. Currently federal workers may retire at age 55 after 30 years of service, on a pension equal to 56 1/4 percent of their highest three-year salary average. The White House says that the average age of retirement is 60.

The budget says that the proposed changes, if approved by Congress, would reduce the federal budget deficit by $15.6 billion during the next five years. Put another way, that means that benefits to employes and retirees would be trimmed by the same amount.

Under the White House plan, the new retirement age would apply to all workers who have not celebrated their 55th birthday by the time the change is made. Anyone covered by the new rules would have to work until age 62 to get unreduced benefits. Pensions would be reduced 2 percent for each year before the employe's 62nd birthday that an employe retired. Benefits also would be lowered by basing pensions on the highest five-year average salary.

The budget also would skip the 1987 cost-of-living raise for retirees, and put them on a system giving them future raises that were 2 percent less than the actual rise in living costs.

Because of congressional actions, federal retirees have received only one raise -- 3.5 percent in December 1984 -- in the last three years, although inflation during that period was up 10 percent. The 3.1 percent raise retirees that were due last month was taken back because of the Gramm-Rudman-Hollings budget law.

Officials pushing the pension changes argue that the early retirement feature is a luxury most private workers don't enjoy and one the government can't afford.