Steps to merge cells

1. select desired cells
2. click on FormatCells. Format cells window should be open now
3. click on the Alignment tab
4. check Merge cells under Text control
5. click on OK

Steps to unmerge cells

1. select desired cells
2. click on FormatCells. Format cells window should be open now
3. click on the Alignment tab
4. uncheck Merge cells under Text control
5. click on OK