Depends on location
If the cold sore
is in a place that doesn't make contact with your mouthpiece, you can still play.
You may want to see your family doctor to consider getting on medication for your cold sore, which would speed up healing.
If the cold sore is on your lips, best advice is to lay off for a week or so, and continue to practice your music holding your horn up, breathing, and fingering with a metronome.