This command re-labels residue numbers in each chain (i.e., segment) so that they start with renumber_residues[iseg]. In addition, the single character PDB chain id's are also assigned: They are obtained from the corresponding elements of segment_ids. Thus, there should be as many elements in segment_ids and renumber_residues as there are chains in the current MODEL.
# Example for: model.rename_segments()
# This will assign new PDB single-character chain id's to all the chains
# in the input PDB file (in this example there are two chains).
from modeller import *
env = environ()
env.io.atom_files_directory = ['../atom_files']
mdl = model(env, file='2abx')
# Assign new segment names and write out the new model:
mdl.rename_segments(segment_ids=('X', 'Y'))
mdl.write(file='2abx-renamed.pdb')