I believe Preston makes a "flush" and a "cleaner" which is a bigger container.
My suggestion is to dump your system...be environmentally responsible.
Remove your t-stat and reinstall the housing/house.
Remove the upper a lower hoses at the radiator.
Sticke a garden hose in your upper hose and let 'er rip...let fresh water circulate through your cooling system. Stand back because you won't believe the stuff that comes out. Turn your heat control selector to HOT while this insanity is going on...
You can flush the radiator with water by sticking the hose in the fill neck. Also check to see if the radiator core is corroding. If so you should replace.
Reconnect your hoses, dump the Prestone cleaner in and top off with water. Run the engine for a few minutes to ensure you bleed the air. Top off and leave your heater on.
Drive for 6 hours ... total not straight. When you're done, cool down, dump and dispose of the fluid being environmentally friendly
Install a new t-stat, fill with coolant, run the engine, wait for the t-stat to open, top off with coolant, replace the radiator cap and you're done.