Mattress Warehouse / SleepHappens.com review: mattress/applauiifm. f. matt

I purchased the above mattress set on 8/17/2016. I thought it was quite comfortable for a while. About six months ago, it began to sink in the middle. Since I am the only one who uses this mattress, and I am 77 years old, I do not understand why it is sinking in the middle. I lay on the right side of the bed and there is no reason for it to sink in the middle. I feel that this mattress should be replaced because it should have lasted a lot longer than this. I would also like to know just what the warranty is and how I can get a refund or a replacement.
Customer #1017101 order #1221330