Apple - Is there a keyboard short cut to edit a cell in Numbers?

(⌥OptionEnter↩).

Or just return if you've unchecked Return key moves to next cell in the inspector:

There are two ways to change the content of a cell using the keyboard:

1. Navigate to the desired cell with the arrow keys and then simply start typing. The previous content of the cell (if there was any) will be replaced.
2. Navigate to the desired cell with the arrow keys and the press ALT+Return. The content of the cell will not be replaced and the cursor appears at the end of the cell content.