How to fix or make a perspective image flat in Adobe Photoshop? And how to do a screen replacement in After Effects?

1. Edit > Perspective Warp.
2. Draw the plane/corners
3. Switch to Warp
4. Redraw a flat plane to map the image correctly.

Reference:

https://helpx.adobe.com/photoshop/using/perspective-warp.html

Screen replacement video: