Built In module Faceplate editing
Aspects belonging to closed libraries can not be modified, eg AC 800M's SignalLib.
Modifying standard has several drawbacks complicating future upgrades, etc.
A middle way could be to add an own extension library and *copy* desired aspects there. For a custom faceplate I recommend creating a custom and new category of faceplate type so that you can put it first in aspect precedence order (which controls what a left click in graphics, etc does).
Extension libraries makes it a bit easier to handle upgrades, etc.
You can't edit/modify the default faceplates of the system libraries since these are locked by the system.
if you want to edit the default library faceplate then copy-paste the aspect in the object type str-respective object& rename the same & edit/modify, then later you make this faceplate as a deafult aspect for that obect types.