Your warranty is a manufacturer warranty and is not issued by the retailer. However, Mattress Firm acquired Mattress Giant, so stop in any mattress firm in your area and they'll give you the warranty forms to fill out. They'll have an inspection company come out to do an inspection of your mattress to see if it qualifies. There is a fee for this and it can take several weeks. It will help if you still happen to have your receipt.

Generally speaking, for innerspring beds if there's any sagging area greater than 1 1/2" in the mattress than it is considered a warranty issue. It's a bit different for memory foam mattresses, usually anything greater than 3/4" is covered under the warranty.

Good luck!

The retailer you seek help from will likely send out a 3rd party to inspect your mattress. They look for 3 things, proper center support from the frame of the bed, matching set of mattress and boxsprings, and the last is to make sure it's not stained. Any sagging or dipping will need to be 1.5" or deeper to qualify for warranty coverage if the three requirements above are met as well.

Please post a follow up with how easy or difficult the process is for you. It would help a lot of shoppers who reach this forum through their research.