What's the best way to drain it? And how do I get rid of the air bubbles and how do I know I still have air bubbles left?
All good. But yeah as said above literally unskrew the cap and put the new one in, I tend to not run them though. Just one less thing to go wrong.
Hence why you let it warm up for 5 minutes or so. Then it'll be hot. To be honest it's just my personal preference. One less thing to go bang.