As the scorer, you are responsible for recording libero replacement. Here are instructions on how to do that:

Putting the Libero In 

1. Tap the libero (it is then highlighted in red).

2. Tap the player the libero went in for.

Taking the Libero Out

1. Tap the libero (numbered in red) on the court. 

2. Liberos who rotate to the front row are automatically taken out.