What is the best way to do this and how do I avoid air in the system? Thanks.

The proper way to do it would be to drain the system completely, using the tap next to the bottom of the radiator, and - having tried various shortcuts - I wouldn't bother attempting anything like clamping the hoses so you only need to empty the tank (although you might be more successful with it than me???)

Once you've finished, fill up the system, but leave the cap off. Start the engine and run it at a fast idle until the radiator fans have kicked in and gone off again. By this time, you should see coolant flowing into the expansion tank from the return line. Turn the engine off, leave it to cool, top up as required, and replace the cap!

i wouldn't drain the whole system , just drain it down so the reservoir is just empty, then do your swop over , refill reservoir to marks , and run up engine

if you do it this way , you should not have any air in the system, as the reservoir is the highest point

Job done. Just drained it enough that the bottle was empty. Sadly fumble one of the screw that hold the bottle in place as I was removing it. No idea where it landed. Had to make to with a random screw from my toolbox when fitting the new bottle. For the first time in ages I have no alarm when I start the car

Sadly fumble one of the screw that hold the bottle in place as I was removing it. No idea where it landed. Had to make to with a random screw from my toolbox when fitting the new bottle.

You should see the length of the bolt I had to use to replace the 'impossibolt' holding SWMBO's battery in!!! Glad it's sorted, though...