1. In the Unreal Editor, just above the viewport, click on the "World Settings" button.
2. To the right, below the Scene Outliner tab, you should now have a World Settings tab open. Navigate to the "Physics" heading and check the box next to "Override World Gravity."
Move the "Global Gravity Z" slider to the right to decrease the gravity, and to the left to increase the gravity (it seems a bit counter-intuitive, but the higher the value, the lower the gravity).
That's all there is to it. If you came here looking for a tutorial on how to change global gravity settings dynamically at runtime using blueprints, stay tuned. Right now there doesn't seem to be an easy way to manipulate world gravity settings at runtime without using code, but you should be able to achieve the same results with a physics volume.