You can use the OSC message:
/vcs/map 131 <program number as float>
to send a program change for MIDI channel 1.
For other MIDI channels, use (channel - 1) * 256 + 131 for the first number. For example, for channel 11, you would use:
/vcs/map 2691 <program number as float>
Note that the first argument must be an integer and the second number a float.