Several dedicated video converters like the Freemake Video Converter should be able to help you convert h264 to h263.
Except for that you can use FFmpeg encoder and command line tool for Windows, Mac and Linux to convert H.264 video data to H.263 video format, which is what h264 to h263 conversion represents. Install FFmpeg and type to command line:
ffmpeg -i /path/to/your/file.h264 -vcodec copy -acodec copy /path/to/your/newfile.h263
Transformation H.264 videos to H.263 format can decreased video and audio quality.