To change the Placement ID of a Motiontracker, you first need to start up MVN Studio.
Open the Configure session window by going to "File -> New Recording Session" (Ctrl + N).
Make sure you leave this window open, otherwise you will not be able to change the ID.
Now open the hardware list to show the connected motiontrackers.
Click right mouse-button on the motiontracker from which you want to change the ID and set it the the desired location.
To ensure that the changes are saved, go to "Tasks ->Reinitialize suit(s)".
Tip: You can also assign a tracker to a different segment if you drag and drop it to the ID to the Location, or vice versa.