I understand that there is a way to change the squadron code letters and numbers on the side of the Spitfire. Where, in the mission editor can you do this? Thanks.

In mission editor choose any plane and see the right side panel.

... and in that properties panel, click the "tail #" window and type in eight characters (without dashes or spaces). First three will become then squadron letters, remaining five will show as RAF serial number.

I'm almost embarrassed to admit that with all the years that I've owned DCS, I never knew about that!

Truth to be told, this extended functionality was introduced last year for the Spit. Previously the "tail #" window was there, but it allowed up to three digits only I think?

Edit: mind you, just like with other planes, some custom Spit skins made by the community have fixed serials and codes and you won't be able to change these, not in the mission editor at least.