use a mission editor, you can find this tools here on simviation downloads or on the combat flight simulator site from microsoft (manuals are most of the time included). The one I use is that from Cor Dikland

You can do it also in notepad by editing a copy of the ".mis" files you find in the campaign and mission folders of your combat flight simulator folder.

In both cases make sure it is a copy that you are editing or you will loose the original mission.

To edit a campaign you can edit a copy of a ".cmg" file you find in the campaign folder. You find the new campaign in the custom campaigns. It is easy to figger out how it works.

You can also download many freeware campaignes and missions if you got bored with the stock ones, just check them out on the downloads here and on other sites.