i want to replace background with still image/picture.
and keep the foreground? Then chroma keying is the best way, but if you did not shoot with that in mind, you are in for a lousy time. Rotoscoping in AE may be the only way and that often means editing each and every frame using mattes, apart from reshooting.
The problem is that you only have one video image consisting of foreground and background together (in a single dimension, depth) and you want to separate them. If you did not shoot with a green screen, it is nearly impossible. If you ask about this then rotoscoping is practically out of your league and you have two realistic options left, reshoot with a green screen and chroma key the shoot, or forget it.
I have to agree with Harm on this one. If you have to ask how to do it, then you really can't do it with the footage you have.