If you already cashed the check, you're done. That means legaly you accepted their offer. Otherwise, you can get a lawyer and have him take care of it.
You must prove that you suffered more damages than what they are paying you. This can often involve suing them, although you can sometimes convince them if you have proof. Hiring a lawyer can also help.
Often isurance companies try to minimize settlement payouts. The only way to fight them is to hire a good lawyer who specializes in insurance settlements and sue them.
The first thing to try is to negotiate with the company, telling them that you do not accept the settlement they propose. If it is allowed in your contract, you can hire a lawyer and take your insurance company to court.