How and where do I reattach hose that has fallen off from coolant reservoir tank on 1995 GMC Jimmy?

There should be two hoses which attach to this tank. Find the one that's disconnected... that's where it'll reattach. How... that depends. If it's just a clamp which came off, you go to NAPA or wherever, and buy a hose clamp... get the ones that you tighten down with a screwdriver or an 8mm or 5/16" socket... don't get the self tightening ones which you just use a pair of pliers for.. those are junk. If the piece which the hose attaches to came off the reservoir tank, you'll have to replace the reservoir tank.