[SpecB]

I'd drop the coolant from the radiator rain plug (possibly disconnect bottom hose to speed up drainage) then refill the system - I usually use the top radiator hose as per this method:

Start engine and let it start to warm up, set heater control to full hot ad run engine until it starts to blow hot (if possible).
Stop engine and allow to cool slightly.
Drain System (Open cap and drain from either radiator drain plug or disconnect bottom hose). Once drained close drain plug/reconnect hose!
Disconnect top radiator hose from Radiator side then pour water into the hose to very slowly fill the block side.
You will hear the Raditar start to fill, as soon as you get coolant at the top of the radiator (it will come out of the top hose connection) reattach the top hose.
Fill header tank VERY SLOWLY - some people recomend using the turbo coolant line at the rear of the tank pipe to fill the system, I generally just slowly fill the tank.
Start engine with header cap off - slowly top up though header until it wont take any more and starts to over flow.
Refit cap and make sure there is some coolant in the overflow tank to prevent air being sucked back in as the system cools.
Run the car up to temp and allow to cool, check levels and top up as required.