Step 1: Select It
Drag your mouse over the values you would like to change the format of. There is no limit to the number of cells you can format but they will need to be next to one another in order to be selected as a group.
Step 2: Click on Custom
From the formatting menu at the top of the interface select the Custom option at the very bottom of the dropdown. This will bring up an entirely new window that you can use to select values.
Step 3: Pick your value
Use the drop-down menu at the top of the new window to select the value type that you wish to change. In this example we are using Date & Time. The values beneath will now change.