Pressing the CommandCtrl+Enter keys inserts a manual line break. This shortcut works directly in the cell or in the input line. The input line can be expanded to the multi-line by the Down arrow button on the right.
If you want the text to automatically break at the right border of the cell, proceed as follows:
Select all the cells where you want the text to break at the right border.
In Format - Cells - Alignment, mark the Wrap text automatically option and click OK.