Adjacent table cells can be merged and unmerged.

These options only appear when you have selected multiple table cells or a previously merged table cell.

Merge two or more table cells

1. To select two or more table cells, hold down Shift and click on multiple adjacent cells in the table.
2. In the Arrange tab of the format panel, click on Merge.
   

Unmerge merged table cells

1. Select the merged cells in the table.
2. In the Arrange tab of the format panel, click on Unmerge.
   

If you are using the Sketch whiteboard-like editor theme, you can merge and unmerge table cells via the Table tool in the toolbar on the left.