Dear Arora ,
Your question itself is not clear . It is presumed that you are a Punjabi and you fell in love wtih a girl from Rajput because her name is Singh . Because you are from Punjab , her father - being a Rajput , might be worried about the repercussions his hamily mahy have to face by your marriage . Real love does not call for change of religion, caste,nationality - however, if you so desire to change your caste , go to the panchayat office , get the form and fill it with an undertaking and other realted procedures. You will have to advertuse in the nespapers about your changed caste - that is all . This is only an administrative procedure and there must not be any objection from others for cchanging your caste .
Go ahead, change your caste, get her father's consent , marry her and lead a happy married lovers' life. ALL THE BEST!