Saggy Bed Cheap Box Spring

I purchased my queen size set from Mattress Giant, I needed a firm mattress. Well the mattress I really don't have a problem with except it is starting to get indentations, but my box spring is broken! almost to the floor. I purchased a new box spring, the sales manager tried to tell me to get a split box but I did not listen. I got a regular box spring cannot get it up the steps! The boxspring been sitting in my living room for almost three years. I got the first one up the steps because the banister was off the steps. Mattress Giant refused to take the box spring back, now I am stuck with an unused box spring and a my mattress is getting saggier by the minute