Hi Vinit,
There should be a property called "lisa.REST Step.http.headers" where "REST Step" is the name of your test step which should contain the headers. The type of the object contained in this property is a com.itko.util.ParameterList. Here is a sample script that would allow you to get one of the values:
import com.itko.util.ParameterList;
import com.itko.util.Parameter;
ParameterList list = testExec.getStateObject("lisa.REST Step.http.headers");
Parameter p = list.get(0);
return p.getValue();
Josh