Go to the edit page of Bird-6740, down the right hand side you will see the father listed. Underneath his name, there is a replace/remove link. Click that, then either
add the profile number of the new father in the box at A, or
scroll down and click to remove the current father, if you don't have a new father to add yet.
You will need to add a father in order to link the two brothers. If you don't know his forename, you could add Unknown Bird as the father to the two, and add his forename later when it is known.