what, to make it short-sleeve, or because the arms of the shirt are too long for your actual arms?
If it's the former, cut the sleeves at the elbow and do the gig and then get it tailored...
if it's the latter, pull a "Bobby Brady" and hang by your arms on some monkey bars until they stretch...just think: 1st position won't be much of a stretch anymore.
Or, buy a tailored shirt.