Finally I was able to found the parameter names where in each an every block.So we can change any parameter using set_param command except drop down menu.The thing you have to do is right click on any block that you want and select the 'View Mask'.Then it will appear the mask edit window.then select the 'Initializing' tab and then all the parameter names will show under the 'Dialog variables'.
It will show you that the parameter names are not in real full names but they used short name instead.If the mask can be edit, we can use any name instead of the prevailing name.Here is the set_parm command;
set_param([model_name '/Block_name'],'parameter_name',value)
for QAM block to set the constallation parameter;
set_param([model_name'/QAM Modulator Baseband'],'SigCon',value)