Merge cells in Google Colaboratory

There's no such command in the menu. But if you look at Tools > Keyboard shortcuts...

You will see 'Merge focused cell with next cell' which you can set your own shortcut key combination.

You can CTRL+Click the cells you want to combine. Then, right-click one of the highlighted cells, and then click "Merge selected cells"

Go to Tools->Keyboard Shrotcuts--> and then find "Merge selected cells" and also you can choose your key's combination too.