You should renumber them before loading them in Pymol, otherwise you might get overlapping residues and lose information. To do it Pymol, you use the alter command, e.g.,

alter your_selection, resi=int(resi)+1-int(resi)

This would renumber the selection: 1..2..3...etc

Alternatively, to manipulate numberings outside Pymol rather quickly, you can use this set of scripts. pdb_reres.py takes a PDB and a number as input, e.g.,

pdb_reres.py -1 astructure.pdb

This renumbers that structure, sequentially, from 1. Pymol is 'smarter' in the sense that it keeps gaps in the numbering if they exist.