how to change format of ent file into PDB format?
Just rename the file to .pdb - in other words, .ent files are PDB format:


  The archival PDB files will be distributed with the reserved conventional names, in the form pdbentry_id.ent, where entry_id is a PDB 4-letter code, e.g. pdb1abc.ent, for PDB format entries; rentry_idsf.ent, e.g. r1abcsf.ent, for X-ray experimental data; entry_id.mr, e.g. 1abc.mr, for NMR experimental/constraints; entry_id.cif, e.g. 1abc.cif, for mmCIF format entries; and entry_id.xml, e.g. 1abc.xml, for canonical XML format entries.


From http://www.wwpdb.org/about/faq