Heat it up to make it expand, or cool it down to make it contract. When you heat a gas, the particles of the gas get farther apart, therefore making the gas expand. The opposite happens when you make it colder.
People are going to tell you about heating or cooling the gas, but there's only one way to change the volume of a gas: Put it into a different container !
A gas always expands to fill whatever you keep it in ... doesn't matter how warm or cold it is. So the only way to make it change its volume is to put it into a larger or smaller jar.
