How do you change a law??

That depends upon which law you want to change and who you are. If you're Joe Citizen, you can petition your legislator (state rep, state senator, US rep, or US senator) to introduce legislation changing the law that you want changed. In some states, citizens can bypass the state legislature and put a proposition on the ballot if they get enough signatures. If the proposition passes, then it become law.

If the law is fundamentally flawed (e.g. it violates a more supreme law such as the constitution), you can find a way to gain 'standing' and challenge the law in a court.

The Senate and the House would need to vote on an amendment in order to change a law that is already in place.