Plastic home button fell off

I was using my iPhone 4s and suddenly the home button fell off it is only the plastic top , the under part works fine , I need a way to fix this, please note that I cannot open the phone or unscrew it whatsoever because I have no experience with the matter and I am afraid to ruin something, furthermore I am really low on budget so I can't take my phone to a professional or Apple , I would be so thankful if there was a solution to this issue and if glue works please tell me how to get it done , thank you so much in advance