First, you must highlight the cells you want to merge.
Then right-click and select "Format cells". Or you can click on the "Format menu" and click on the "command Cell".
In the Alignment tab, check the box next to "merge cells". Click "OK".
In the merged cell are only those data which were in the upper left cell. Therefore, never merge cells that already contain information.
For maximum convenience, you can make a special button merge cells toolbar. Go to the customizing the toolbar (View – Toolbar – customize). In the opened window, click "Commands" in the left column select "Format" and from the right list drag the button "merge cells" on the toolbar.