Method 1:  The easiest way

1. Click and hold the green expand icon  until you see the split view come up (don't let go yet). 
2. As you hold the button, you can drag it to the left or right side of the screen and release.
3. You will then see Mission Control where you can select your next app you want to display

Method 2:  

1. Control+Command+F will enter Full Screen mode
   
2. Use mission control to drag your app to the left or right side of the screen and release.
   
3. You will then see Mission Control where you can select your next app you want to display

The above instructions are provided by apple support:  https://support.apple.com/en-au/HT204948