To enter or change the name of the main goal or any other goal in the goalscape, simply select it, click in the name field in the yellow title bar at the top of its panel and enter text.
You can also click the goal twice (not as fast as a double-click): a pop-up text field will appear in which you can change the goal name. When you have finished entering text, click anywhere else on the screen or press Return. Do not use Return in the middle of a goal name (to force a line break, say) as that may result in losing text.
Alternatively you can access this pop-up name field by right-clicking a goal and selecting Rename Goal from the list of options. Note that right-click is context-sensitive so it applies to whichever goal you right-clicked, whether or not it is selected and highlighted.
As you enter goal names in these fields, the text appears simultaneously in the appropriate goal slice in the goalscape.
All alphanumeric characters and most other keyboard symbols are permitted in goal names. Note however that the first 'Save'/'Save As' and all 'Save Selected Subgoal As' operations use the relevant goal name as the default file name. Since certain characters (eg "/") are not permitted in file names, they may cause a problem if used in goal names.
There is a 100-character limit on goal names.
The text size of the goal name displayed on the goal map changes automatically with the size of its slice. So when slices become thin the text becomes smaller. If the slice is too thin, the name may be curtailed or disppear completely; in that case the name will appear on a pop-up tooltip when you mouse over the subgoal slice.
You can change the size of the text manually using right-click functionality, F8/F9, or the hotkey shortcuts 'Ctrl+ -' and 'Ctrl+ +' (PC/Windows) and 'cmd+ -' and 'cmd+ +' (Mac/OSX).
There is a short video about goal names here.