You can get mender-artifact usage help with -h flag, try these:
./mender-artifact -h
./mender-artifact modify -h
Keep in mind that modifying artifacts with mender-artifact is currently a slow process, see https://hub.mender.io/t/mender-artifact-hangs/752/4
In any case, this is one of many ways (as you’ll see later) that different Mender apps propose to accomplish one basic task - changing /etc/mender/mender.conf file.
So the easiest way for POC is to modify this file manually after checking it’s format in docs.