Open up the mission editor. At the top right hand corner you'll see a funny picture of a cloud. Click on it.
This opens up something which is known official as the Intel box. Here you can fix all kinds of cool stuff inlcluding the weather. However, it also allows you to choose the name of hte mission (The box near the top.) This is the name that will appear in the Single Mission list and also on the Plan page in the Briefing.
Note that the description box doesn't really do anything ... you can write text here but it won't appear anywhere else. It is usefulful however for notes to self.