You can replace/change an existing asset (image, video or document) by hovering your mouse over the asset in your library, and click ... then click "Replace asset".
A window will pop up. You can drag in the new file that you want to use.
The new file will be uploaded and take place of the old one.
Every where this image is used (playlist, schedule, ScreenZones, etc.) it will be replaced with the new file.