How Can I Adjust The Seat In My Jogger Stroller?

I bought a Baby Trend Expedition jogger stroller at a rummage sale. I looked at their website and know it's definitely not their newest models.
I cannot figure out how to adjust the reclining seat. Right the seat is all the way reclined and it worked great for my son when he was a baby but now as a toddler, he wants to sit up and see things.
The only thing behind the seat is a strap and buckle but I can't see anyway to adjust this to tighten or loosen it. Anyone have any idea how the seat can be adjusted?