Here is the procedure I found on different forums. Here
https://www.pentaxuser.com/forum/topic/error-a90-2305 in English
-fwdc110p.bin works for the S
-fwdc124p.bin works for the S4i
-fwdc130p.bin works for the S5i
Step2. Unzip it and put it to root of the removable drive of the Optio S (the SD card)
Step3. Press MENU+FlashIcon+ON
The screen will show
++ KB341 ++
PR : 03.02.27.16.34
LD :2.00
MI : 111
Step4. Press MF twice
Step5. Press Right OK twice
Step6. Press MENU
The screen displayes Menu with 11 options
Step7. Select ROM UPDATE using the down key
Step8. Press OK
Step9. Select SYSTEM INITIAL
Step10. Press OK
Step11. Again OK
Step12. Switch OFF
Step13. Press MENU+ON
Then the dialog "Program Update (Yes, no) appears
Step14. Use up key to select: YES
Loading message comes on screen followed by
COMPLETE
Step15. Switch OFF the camara
Step16. Switch ON