ok....
Go to W's Contact.... U will see 2 numbers in that, of which one is B's.
Now click on the edit option under W's contact (Hope you know how to navigate to that option).
Once you edit, go to the location on the screen where you find B's contact number.
Do a long press (keep pressing on the location for a few secs) ... you might see a few options like 'Select all, copy all etc'.
If you see the option copy all, click on that...
Now you have copied B's number.
Exit W's contact.
Follow a similar step until edit for B's contact.
Click on the blank location next to 'phone no'... Do a long press and youll see a paste option. Now paste it there.
I know this stepwise procedure looks a bit childish, but this is the only way i could explain it
.
Also make sure that you delete off B's number which is still present under W's contact details by using the delete button on the keyboard.
If you still facing the issue, just let know where exactly you facing it.
Cheers