Drain the engine coolant until the overflow tank is empty.The drain plug is on the right lower corner of the radiator (driver side).If your coolant reservoir is empty already, skip this step.
Use channel lock pliers to disconnect the hose that connects to the bottom of the overflow reservoir.
Disconnect the return line at the top of the reservoir. You will need to press the clips and pull the line away. Use a flathead screwdriver to release the reservoir from the metal brackets. Lift up the reservoir and remove it.
Install the new reservoir in reverse order.
Reconnect both hoses.
Snap the new reservoir in its place.
Add 50/50 engine coolant,
Reinstall the cap.
It is recommended to replace the coolant reservoir cap at the same time you replace the coolant tank.
That's it. Replacing the coolant overflow (expansion tank) on Ford Focus is very simple and takes about 30 minutes.