ANSWER:

You must advise them diplomatically with good manners, knowledge, and wisdom and never cross the line of respect. However, you must never compromise your religion or give in to their pressure. 

I am aware that we have to respect our parents & Allah says be humble never say uff.If parents & grandparents follow cultural tradition like kids should bend down & say Salam, going to graveyard & putting flowers on grave. I have noticed if I try to say that its not right thing to do they get upset saying now kids will teach us right and wrong. How to deal in such cases pls help.