You can edit the majority of individual text boxes in the chart editor, although I do not believe it is possible to use super-scripts or sub-scripts. If you need them, the best advice I can give is to export to some vector format and use an appropriate editor (e.g. Inkscape, Illustrator). With versions 17+ I've had good success exporting to emf for these purposes.
Frequently you can get the appropriate labels in charts without post-hoc editing by specifying meta-data.
Andy