If you want to change the default screensaver, follow the steps below: 

1. Swipe down from the top of the screen and tap the Settings icon. 
2. Tap Display & Input. 
3. Tap Screensaver. 
4. Tap More. 
5. Browse through files to select a new image you want to use as the new screensaver. 
6. Tap the image to preview it. 
7. In preview mode, pinch to zoom in and out of the image freely with two fingers.  You can also move it around with one finger or your pen.
8. Tap Fit or Fill to adjust the image. 
9. Tap OK to apply the image as the new screensaver. 

Note: The supported image formats available for screensaver customization are PNG, JPG, and JPEG.