If the first residue in an object called "name" is 4, to change this to 1, write in PyMol:

alter name, resi=str(int(resi)-3) (this will subtract 3 from each residue number)

To renumber a residue range defined by the 'sele' command (e.g. sele resi 234-310), type:

alter sele, resi=str(int(resi)+x) (where x is the offset to be added to each residue number)

With the same command, you also can rename a chain. Select group of residues and then type:

alter sele, chain='A' (chain A is defined)