If we're talking a plastic toy helmet, then a $40 heat gun from Home Depot will do wonders. You basically have to gradually heat the helmet slowly until it starts to slightly sag under its own weight. Have the faucet running cold water, then pull the sides apart to desired width, and then cool it rapidly under the faucet. (If you simply push the front and back towards each other, the sides will widen out).
I've done that to the Rubie's Supreme Darth Vader helmet to adjust the right side's neck flare, and to reduce the overall width of the rear opening.
Do not hold the heat gun too close, otherwise the plastic will start to melt and blister.
Heat can sometimes alter the appearance of the plastic. For example, if there is a slight granular texture, that texture might melt. In some cases you may have to refinish the helmet.
Always heat a larger area than the immediate area you're trying to shape, else the helmet may crack.
You can do this incrementally and slowly until you achieve the desired results.
Hint: if you have a range top with fans, turn the fans on to draw any of the plastic vapors away from you.
