Are you asking how to set the tempo in Vegas? File / Properties / Ruler ... enter Beats per minute there.

Are you asking how to change how fast the audio plays? Right-mouse click on the audio clip, bring up it's properties, set the Time stretch / pitch shift to either "Change length" setting. If you select preserve pitch then the pitch won't change with the speed and vice versa. Enter in the appropriate new length. What this new length should be depends on the original tempo. If the song is currently 120bpm and you want to change it to 136 then it will speed up by a factor of 1.1333. Divide the original length by this factor. This caculation will probably be easier if you set the ruler time format to seconds first.