The File name is different according to the BlackBerry model you choose, just in case. Not sure how different this is from OS 4.5.0.161, so if you notice any differences let me know in the comments!

BlackBerry Handheld Software v4.5.0.255 (EastAsia)

Package Version: 4.5.0.255
Consisting of:

• Applications: 4.5.0.162
• Software Platform: 2.7.0.103
• File name: 8100EastAsia_PBr4.5.0_rel255_PL2.7.0.103_A4.5.0.162_Vodafone_Essar_Limited.exe
• File size: 89.67MB

Note: The Software Platform and Applications version numbers can be found under Options-About screen on the handheld.

• Download: OS 4.5.0.162 for the BlackBerry Pearl (8100,8110,8120), Curve (8300,8310,8320) and 8700 from Vodafone Essar Limited
  
• How To: Upgrade your BlackBerry Operating System (OS)

REMEMBER: Install at your own risk. Always make backups before making major changes/upgrades/downgrades to your BlackBerry.