Children with autism sometimes have a difficult time with emotional regulation and calming down once they are upset. Three basic calm down strategies are:
1) counting (counting to 10, counting down from 10, ...)
2) taking a couple of deep breaths
3) tensing and releasing (make a fist and release, curl toes and release, ...)
The tricky part is to get an upset child to do those things. For young children, I have had good luck putting things to music. The above song works well. It combines the 3 strategies above when taught using hand motions. Start by holding up 5 fingers and then blow an imaginary flame out and put the finger down with each verse.
It is best taught when the child is happy. If the child does not want to do the hand motions, then you can do it and provide them an opportunity to blow out your candle. If they don't want to, don't push it. I will say, "that's okay, I got this one," and model it for them. It's important to keep it fun.
I also provide praise for any participation (i.e., "good job", "nice work" after each candle is blown). When things seem bad enough to a child that they are emotionally distraught, hearing that they are doing good at something helps turn it around even faster.