Rebalance for an Effective Defense Uniform and Civilian Employees Act (REDUCE Act) - Limits full-time positions in the Department of Defense (DOD), in each of FY2021-FY2025, subject to specified adjustments to be made by the DOD Secretary, to a number not greater than 85% of the number of such positions at DOD as of September 30, 2015. Permits not more than 1,000 of such positions to be career appointees within the Senior Executive Service. Authorizes the DOD Secretary to offer voluntary separation incentive payments and voluntary early retirement payments to achieve the reductions required by this Act, but requires the Secretary to use involuntary measures, including reductions in force, beginning on October 1, 2015, to achieve required reductions in personnel levels if voluntary measures are inadequate.