High-critical-temperature $(\; T\; c)$ superconductivity at 48 K is reported for hydrogen-doped SmFeAsO epitaxial films on MgO single-crystal substrates. The key processes are pulsed laser deposition to grow undoped SmFeAsO epitaxial films and subsequent topotactic chemical reaction using $\mathrm{Ca}{H}_2$ powders under evacuated silica-glass ampule atmosphere. Based on this post-deposition thermal annealing treatment that we have developed, a maximum hydrogen concentration $x=\sim 0.35$ was realized in $\mathrm{SmFeAs}\left(O_{1–x}{H}_x\right)$. Disordered hydrogen substitution at O sites is experimentally confirmed directly by atomic-scale microstructural observations. Magnetization measurement validates the bulk nature of the high-$T_c$ superconductivity in the films. This method will become an effective and general method to fabricate various high-quality oxyhydride epitaxial films.