To make any changes in the table you should open it in edit view by pressing Edit table icon on the toolbar or by double clicking on any cell or by selection the Edit link from the context menu.
Initiate cell edition you can from edit view by
When cell editor is active you can save new value and close editor by pressing Enter or clicking on any place
of the page outside the editor (the only exception is Multiline editor - to close it with saving entered value you
need to press Ctrl+Enter)
To close cell editor without saving press Esc.
One of the editors is text editor. It understands Alt+Enter (Ctrl+Enter for Opera) key combination to switch to multiline editor.