Show off in front of her
wait until she breaks up with her boyfriend and then make your move
you make a girl jealous and the girl will hate her boyfriend and follow you as her boyfriend and listen to that song Justin bieber one time
be there for her when her boyfriend isn't girls love guys that are always there whenever they need them =)
If you like a girl hug for a wile and then make your move.
Then she has to choose you or her boyfriend.
If u like a girl who already has a boyfriend but you dont know if she likes you or know,the first thing you have to do is that you will have to make friendship with that girl and if she is already your friend,then increase your friendship and one day confidently ask her that what does she feel about you..
If the girl in question likes you then you won't need to convince her to let you be her boyfriend, you must wait for the signs to see if you think she is interested, let the girl make the move, don't be pushy, be patient.
Does the girl know you like her? How long have you an and your boyfriend been together? And are you asking what should you do?
Try an show her how cool you are compared to her boyfriend
Wait till she doesn't have a boyfriend or move on
If he is her boyfriend, hopefully, she would like him.
Treat her better than her boyfriend does.