Tips on how to replace a freewheel on the bike
Grabbing the cogs is no good as you they spin on the bike freewheel without turning the central mechanism that is screwed on the hub shell. You need a special key to fit into notches or splines on the bike freewheel to turn it. Bicycle Spokes Suppliers
If you have a bench vice you can clamp the key in it, drop the wheel gently onto the key then turn the wheel to unscrew the bike freewheel.