In order to change the lighting in a rendered image by adjusting multichannel EXR settings, you need to first ensure that your lighting is set up properly when building your project.
Click on a light in your project in the Layout window and its settings will appear in the Property Grid on the right side of the window. In the Group section, click on “Click Here” (next to “New”), and then on the 
button that appears in order to create a new group. To select an already existing group, click on “Default”.
Once you have each light in your project separated into groups, you will be able to adjust the brightness of each group of lights in your rendered images. We recommend, however, NOT creating too many different channels (groups) to avoid creating a very large .EXR file. It’s not recommended to create a specific channel for each single light, as this will result in a very large file on your computer.
If you do not create new groups, the program will automatically leave all lights in the same channel; the second channel available contains the light from the sun.
For more on adjusting lighting with "EXR Light Settings", please see the article on Light Compositing.
This article is valid for the 2018 version and later.