Thanks for support in scn.
Finally,I got the solution.Now i can able to change the Fiori Lanchpad .
var objShell = sap.ui.getCore().byId("shell");
var shellheadUseritem=objShell.getUser();
shellheadUseritem.attachPress(myfunction);
myfunction()
{
var userPrefrence=sap.ui.getCore().byId("userPreferencesButton");
userPrefrence.setVisible(false);
}
Thanks & regards,
Durga Rao.