Instructions
Updating Through the Internet
Turn on your PS3 as you normally would.
Navigate to "System Update" in the XMB (Cross Media Bar) menu.
Select "System Update."
Choose "Update via Internet." The PS3 will now download and install the newest system software.
Update Through Storage Media
Turn on your PS3 as you normally would.
Navigate to "System Update" in the XMB (Cross Media Bar) menu.
Select "System Update."
Select "Update via Storage Media."
Select the storage media or game disc with the system update you'd like to apply. When asked for confirmation, select "OK."
Update through Game Disc
Insert the game you'd like to play. This is presumably the one that requires system software version 3.55 to play.
Navigate to the game disc in the XMB (Cross Media Bar) menu.
Press "X" to start the game. The system will now check to see if you have the necessary system software installed. If you do not, it will ask to update your PS3 to version 3.55. Select "Yes," and proceed with the update.