The Sony PlayStation 3 can only be updated with newer firmware, and cannot go back to a lower firmware. There is only one instance when a PS3 can be downgraded back to firmware and that is if the PS3 is running custom firmware (CFW) or is downgrading back to official firmware (OFW). You can safely downgrade a PS3 running custom firmware from the recovery menu of the PS3.
Preparing the Software to Be Installed on the PS3
Download Sony PS3 3.55 official firmware from Sony's software server or from one of the many download mirrors available for PS3 firmware.
Insert a USB pen drive into the computer's USB port and click the "Open folder to view files" option from the "Autorun" window that comes up. A list of folders on the USB flash drive comes up.
Click the "New Folder" option and type in "PS3." Press the "Enter" key to set the folder name. Open the "PS3" folder and click the "New Folder" option again.
Type in "Update" as the folder name and press "Enter." Right-click the firmware file you downloaded and click "Copy" from the context menu that comes up.
Right-click the "UPDATE" folder that was created and click "Paste" to save the PS3 3.55 firmware file on the USB flash drive. Unplug the USB flash drive from the PS3 and plug it into the PS3's USB port.
Installing the Sony PS3 3.55 Software Update From the Recovery Menu
Turn the PlayStation 3 system off.
Press and hold the "Power" button on the front of the PS3 until it turns on and turns right back off. This is triggering recovery mode.
Release the "Power" button and hold it down again. When the PS3 beeps twice, release the "Power" button. The PS3 is in recovery mode. A recovery menu comes up.
Insert a mini-USB cable into the top of a Sony PlayStation 3 controller. Connect the other end of the mini-USB cable into one of the USB ports on the PS3.
Select "System Update" from the recovery menu. Select "OK" when asked if you want to install the software update file found on the USB flash drive. The 3.55 software is installed. Select "Restart System" to complete the procedure.