I got detailed steps from B&W support. It seems pretty easy. I just post it here to share with people that have similar issue. To understand the below instructions, you need to download the service picture from B&W support site. I don't know how to upload pic with the post.
Start by unscrewing #6. Pull out #7 (this is your replacement tweeter) using your fingertips in a forward motion pull the insides out of the housing. Next unscrew #5 this will release the old tweeter. Replace with the new tweeter. Install the replaced tweeter assembly into the housing and press around the outer sides of #7 and hold the reassembled tweeter towards the back of the speaker and screw #6 back on.