In order to achieve this we need to set the "Path to Tcl Library" field with the required version path of Spirent TestCenter installation directory (where pkgIndex.tcl file is located) and uncheck the option "Use paths specified in the TCLLIBPATH environmental variable" in the Spirent TestCenter session profile properties as shown below,
Note: If we specify multiple path in the same field. then iTest will always connect to higher version of Spirent TestCenter.