Check the owners manual for coolant capacity - as for changing it, only needs changing when it drops below spec.
Remove the lower hose with the expansion tank cap off, empty old coolant into a container so you can dispose of it properly. Place a garden hose in exp. tank to flush out system with normal water. Refit bottom hose, open any bleed valves on the radiator or in the pipework to release air and prevent air locks. Fill with fresh coolant and fill tank to nearly the top, and check the open valves untill they start to leak - close them when they do.
Run engine with expansion cap off, and hold at 2500 rpm untill fan cuts in - this will ensure all air escapes the system - some coolant may overflow due to air escaping. With fan working fit cap and leave to idle, recheck fan still cuts in and temp gauge throughout the whole process.
Recheck coolant level next day when cold. Dispose of old coolant correctly