To Edit Your Video
While you can access effects from anywhere, you can use the Effects Workspace to bring it out in the open more. At the top, select Windows/Workspace/Effects.
Click on Video Effects/Keying/Ultra Key and drag this onto the clip in the Timeline.
In the Source Monitor, click on Effect Controls. Then go down to Ultra Key.
Click on the eyedropper next to Key Color, then select the green background.
You can use the settings below Key Color to clean up any leftover green parts in the background or change the setting from Default to Aggressive.
Move the Green Screen Clip onto the Middle Track (V2), then place the clip you want to use as a background below it.