There are some video converting tools available in our converter list, like Free Studio for Windows, or HD Video Converter for Mac.
You can also use a free multi-platform FFmpeg command-line tool:
Install FFmpeg → Open command line (Terminal, Bash) and type:
ffmpeg -i /path/to/your/file.dvr-ms -vcodec copy -acodec copy /path/to/your/newfile.3g2