Can I replace the main breaker in an electrical panel with a smaller one?
You will also likely need to replace the cable with larger gauge wires, though without knowing what's there right now it's impossible to say for sure.  Also, whenever you replace old electrical equipment, you are generally required to adhere to the latest code, so it's possible there are other changes you will need to make too.  This might include AFCI breakers.

If you are unsure, always best to check with a licensed electrician.