Replacing Green Screen / Chroma key
Adobe Premiere uses a tool called Chroma key to made the green screen (or other selected color) transparent. A video or image can then be placed in the track behind the keyed video in the same manner as you would create layers in Adobe Photoshop. Here's how to do it:
In the top right corner, select the Workspaces icon, and then select Effects. This will rearrange some of the panes in your project window.
In the left pane, select Effects to expand the menu. Then select Video Effects -> Keying -> Color Key
Drag the Color Key piece onto the clip you want to apply it to.
Select the clip, and then select the Effects Tab, and select Color Key to expand that section.
Select the eye dropper tool and then select a green portion of the green screen.
You can use the Color Tolerance, Edge Thin, and Edge Feather to fine tune the effect. Parts of the video that have been removed will turn black.
If the video you are applying the Color Key effect to is in track 1, drag it to track 2. You can then place the video or image that you want in in place of the green screen on track 1.