How to convert an mkv file with hevc encoding to mkv with avc
ffmpeg, a command-line tool, can do this:

ffmpeg -i input.mkv -c:v libx264 -crf 23 -c:a copy -c:s copy -fflags +genpts output.mkv


Adjust the CRF for larger or smaller size (lower number produces better quality but larger size)

Download ffmpeg (32-bit static version) from here.