Manually changing symmetry

I have two crystal structures, one in R3m and one in Cm symmetry. Both structures also differ in stoichiometric ratios, ???4 and ?2?2?8, respectively. Both structures look about identical, with the exception of double the amount of atoms in one than the other and the symmetry.

How would I go about manually changing the symmetry of the Cm structure to R3m symmetry? I normally use VESTA as a visualization tool/to create crystal structures, and Avogadro as a tool for imposing symmetry.