My husband changed the "guts" in the toilet tank, but tank is leaking when we flush. Any help would be appreciated?

It is called a tank to bowl gasket. Check to see where the leaking is really coming from. Check the bolts that hold the tank to the bowl, if these are loose tighten. If the leak is coming from between the actual tank and the bowl, then it is probably the gasket. To replace the gasket you need to unhook the fill valve, remove the flush valve, and the toilet bolts, and then the gasket. This is really a pain in the butt. I have done a thousand of them, but you are really better off paying someone to do it. Just my recommendation I wish you luck.

He probably damaged the seal between the tank and bowl. It's basically an O-ring (not really but you get the idea). When the toilet is not being flushed the flapper holds the water in the tank. When you flush the seal prevents water from coming out between the tank and bowl. If this is where the water leaks, you need to replace the seal (being careful not to over tighten).

Most likely the flapper, try a different one. I had that problem.