Simply like this:

ffmpeg -i captions.sbv captions.srt

​YouTube supports only the basic features of SBV (SuBView) and SRT (SubRipText) closed captions. Notably users can download existing closed captions (including auto-generated subtitles) as SBV, convert them to SRT supported by, e.g., MPlayer, edit the subtitles in a text editor such as vim or notepad (hint: YouTube wants UTF-8), and upload the fixed version to YouTube, overriding the old subtitles for the given language.