Depends on the situation! If you're in public, wait until you're somewhere more private to bring it up! If you're at home, say something! There are far too many bad mannered people in the world! It also depends on whom the person is & how well you know them!
You can show them manners by your actions.like if they ask you to hand them something as your giving it to them,say Your welcome.If your walking in a store together you see a lady going in just before you two go a head open door for her.I think you understand what I mean.Even comments about others with out them hearing you.Like Wow that person sure was rude or their parents sure did not teach them about manners.Just a few ideas hope this helps.Always treat this person with respect and soon they will be doing the same to others.Good Luck