Since you are on 4.89, you cannot simply install a 4.88 update file over it; the PS3 will block it as an "older version." You must exploit the system memory (Flash) to allow the installation of older or custom software.
Doing this removes your ability to run homebrew apps. ⚠️ Essential Safety Tips how to downgrade ps3 firmware 4.89 to 4.88
These are generally compatible with downgrading and CFW. Since you are on 4
Plug the USB into your PS3 and attempt a System Update via Storage Media. Plug the USB into your PS3 and attempt
Do you know the or date code of your PS3 to confirm if it supports this process?
The console will display the minimum version it can support. If it says , you are good to go. Step 2: The CFW Toolset Method
Always verify the MD5 hash of your firmware files to ensure they aren't corrupted before installing.