TOPIC:

How to change this icons? I want to replace them with font-awesome icons but I don't know where to look.

site/com_jem/classes/output.class.php

Now I need your help: I wrote where to change these icons and you gave a thanks and a solved! How did you change them to fontawesome?

Do you know how to override the output class?

no. I think, it is not possible to override the output.class.php
But I would like to know how to change these images to fontawesome images.

Its very easy, I do it for example for the submitbutton.
1. Go to the output.class.php file
2. search for function "submitbutton" (there are also functions like addvenuebutton and adduserbutton etc.)
3. search where variable $image is written (eg. line 164).
4. replace the line with the following line: $image = '<i class="fa fa-plus" aria-hidden="true"></i>';
5. Replace "fa-plus" with the icon you want.

Attention: JEM-Responsive uses FontAwesome 4.7 not 5!