To merge a group of cells into one cell:

1. Select the cells to merge.
2. Right-click and select Cell > Merge on the pop-up menu, or select Table > Merge Cells from the menu bar.

To split a cell into multiple cells:

1. Position the cursor inside the cell.
2. Right-click and select Cell > Split on the pop-up menu, or select Table > Split Cells from the menu bar.
3. Select how to split the cell. A cell can be split either horizontally (create more rows) or vertically (create more columns), and you can specify the total number of cells to create.

It is generally a good rule to execute merging and splitting of cells at the end of the layout formatting. This is because some operations such as deleting a column or a row may produce a result difficult to predict when applied to a table with merged or split cells.