Step by step
1. Backup data.
2. If you have root your phone, then unroot first and uninstall or disable any rooted apps.
3. Download Android 6 M firmware for Padfone S from: Padfone S Firmware website. Choose Firmware: V18.104.22.168 section.
4. Copy downloaded file (UL-ASUS_T00N-WW-22.214.171.124-user.zip) to phone’s MicroSD (root folder).
5. Power off the phone.
6. Enter the recovery mode with press & hold power button and volume down button about 4 seconds.
7. Choose recovery mode.
8. Choose apply update from external storage.
9. Choose UL-ASUS_T00N-WW-126.96.36.199-user.zip.
10. After install success, choose wipe cache partition, then reboot system now.
11. Booting will take about 10 minutes.