Return it for a smaller size.
If returning it is not an option, altering is possible..
You can alter the length and width of the sleeves and even the general size and overall length if you wish.
But,
1) the alteration fee will likely cost more, even a lot more, than the cost of the sweatshirt, unless we're talking about a $2000 cashmere Tom Ford sweatshirt.
2) it may not turn out well if the tailor is not skilled or doesn't have a good eye.