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.

As far as YouTube supports these two formats (among others) they are very similar:

The SRT captions start with a counter (increasing numbers), followed by the from --> to timestamps, text (one or two lines), and an empty line.

The SBV captions have no counter and use periods as decimal points in their from,to timestamps.

SRT timestamps have the form hh:mm:ss,nnn, SBV timestamps use h:mm:ss.nnn.

YouTube is reasonably smart enough to detect and report simple errors, but you should make sure that timestamps do not overlap, i.e., the earliest from in the next caption can be the same as the to end of the last caption. Obviously the duration should be not too short, and it cannot be "negative" with a from later than its corresponding to.