In the mid July, 2008. Sony announced the latest firmware update v4.05 for PSP.
Current PSP model is PSP-2000 (thin) version.

Please See the below for more information of download:

Download procedure

1. Prepare to save the update data on Memory Stick Duo™ media using one of the following methods:
   • Connect the PSP® system to the PC using a commercially available USB cable
   • Connect a commercially available Memory Stick USB reader/writer to the PC
   • Use the Memory Stick slot on the PC
2. Create a folder for the update data on the Memory Stick Duo™ media.
   Using your PC, create a folder named "PSP" on the Memory Stick Duo™ media. Within the "PSP" folder, create a folder named "GAME". Within the "GAME" folder, create a folder named "UPDATE". (Folder names must be in all uppercase letters).
   
   
   
   
3. Download the update data and save it in the "UPDATE" folder created in step 2.
   
   Source: us.playstation.com