Models of the upper transition region of sunspots have been derived based on the observed radio spectrum between 3 and 18 GHz from Radio Astronomical Telescope of the Academy of Sciences 600 (RATAN-600) observations. Our objective is to match the spectrum to show that, within the limits of the one-dimensional observations and modeling, we have obtained a reasonable description of the upper transition-region structure of sunspots. We have developed a diagnostic method, based on iterative correction of the temperature-height profile in the transition region and lower corona, and applied it to three selected active regions with unipolar gyroresonance sources. Good agreement is achieved between observed and modeled microwave spectra using one-dimensional, time-independent models in hydrostatic equilibrium characterized by a given temperature as a function of height. We found that above sunspots the upper height of a transition region is located at 2-2.3 Mm, and the temperature of the low corona is about 1.5-2.5 MK.