Change system variables value

Can we change the value of system variables?
if possible then plz give an example
thanx

A system variable is an Form Builder variable that keeps track of an internal Form Builder state. You can reference the value of a system variable to control the way an application behaves.
All system variables, except the following four are read-only variables. These four variables are the only system variables to which you can explicitly assign values.

SYSTEM.DATE_THRESHOLD
SYSTEM.MESSAGE_LEVEL
SYSTEM.EFFECTIVE_DATE
SYSTEM.SUPPRESS_WORKING