There is currently no way to edit text in an already-selected frame other than using the mouse to enter the frame. As everyone knows, mousing is less efficient than using keyboard shortcuts.

Please add a menu command and a keyboard shortcut to edit text of the selected frame.

The most logical thing to do would probably be to use ESC. ESC is already the keyboard shortcut to switch from text selection to editing. It should just be the toggle.

(This might prompt the question: where should the cursor be placed? I would say just place it at the beginning of the frame [not the beginning of the story].)