To merge unmerge multiple cells in one single cell you can use the shortcut keys Alt ➜ H ➜ M ➜ A. When you press the shortcut key merges all the cells but only takes the upper left value (the value you have in the first cell). It shows you a message that says, merging cells only keeps the upper left value and discard the other values”. 
And when you click OK it merges all the cells
Alt ➜ H ➜ M ➜ A
If you want to unmerge cells you need to use the same shortcut keys as above. And following are some other important shortcut keys related to merging cells.
- Alt ➜ H ➜ M ➜ A: To merge selected cells into rows.
- Alt ➜ H ➜ M ➜ C: To merge cells and center align the value.
