This is all in the paperwork that should have come with the knife, but if you send it in to them directly they will not fix or return it to you directly unless you are active duty military or a LEO/fire/EMS. Even if it is a CA legal knife, they still won't return it to you because it is against federal law to ship an auto or balisong across state lines to an individual except in rare circumstances.
The best way to deal with the warranty situation if you are not a person exempted from federal restrictions is to find a local store that is a dealer for Benchmade (or whatever brand it is) and approach them about sending it in and getting it back for you. It is legal for a dealer to do this.
Often times, the best way to find a dealer who trades in autos is to look for one near a military base, the one I have confirmed near me to be willing to do warranty returns is located very close to an Air Force base.