In Microsoft Office 365 Pro on Windows 10, I found two different ways of doing this, depending on what you find easier.
The first is to select all the text in the cell and edit the space before and after for top and bottom in the Layout tab (see screenshot), and adjust the carets on the rulers as indicated above for left and right.
The second option is to move the mouse to the left of the table until the mouse pointer turns to a right-facing arrow and then click once to select the whole table. The Table Tools menu becomes available. Click the dark blue Layout tab under Table Tools (not the lighter medium blue Layout tab for the overall document) and then click Cell Margins. Once the Cell Margins popup is displayed (see screenshot) you can then edit the top, bottom, left and right.