This answer is not specific legal advice, nor does it establish an attorney client relationship. If you are impacted by the law you believe is unconstitutional, you would be able to file an action in circuit civil or possibly federal district court. There is also a possibility of filing a declaratory action in circuit court. However, you do not explain why you think the statute is unconstitutional- which specific provision of the state or federal constitution does it violate, so it is difficult to evaluate if you would have a case.
You would change the law by having a legislator sponsor a bill to change the law, and getting it passed by House and Senate and signed by Governor
Aside from the route of having the legislature itself change a law, you would need to be affected directly from application of the law to have standing to challenge it. Then, you would present your argument to a court and the judge would rule. There are multiple paths for appeal if that is unsuccessful.