[Biopython] help for overwrite a pdb file

[Elisa sechi]

Hi! All !
I'm contacting you in order to ask help about Biopython.
I'm using python,I have extract the atoms coordinates  of a protein from a pdb file and I have used quaternion in order to rotate the coordinates.
I have put its in a new matrix but now the problem is: how do I save the cartesian coordinates in a pdb file???Do I have to create a new structure with the use of builder structure Class??
I ask you if there is a way to overwrite the new cartesian coordinates in the old pdb file that i have used.
Please help me!!!
Thank you very much!
Elisa
   bye