You may change the vertical and horizontal alignment of a cell or a range of cells by selecting the appropriate cell or range of cells, pressing the additional format button located in the toolbar, and selecting the Align tab at the bottom of the popover window.
You will see two alignment controls. The top control lets you choose the horizontal text alignment. You can choose between left, center and right alignment. The bottom control lets you choose the vertical text alignment. You can choose between top, middle and bottom alignment.
Press the Save button to save your changes.