Realme C25s devices started receiving November 2021 Security Patch in the month of December via OTA, it carries an RMX3197.11.A.16 Build number. Although the update fixes bugs and brings stability, this is just an improvement in the security patch without any additional features added.
Similarly, the device had got, October 2021 Security Patch in the month of November, having a similar changelog, like this one.
Realme C25s was released on June ,2021 with Android 11 Realme UI 2.0

Source>> Realme Community
Although it says the update is being rolled out on December 2021, it has the previous month security patch, that's November 2021 Security Patch
| Android OS | Update | Release Date/Status | Estimated Months |
| Android 11 | Out-of-the-box | June 2021 | Nil |
| Android 12 | First OS update | Expected | unconfirmed |
| Android 13 | Second/Last OS update | – | – |
The Realme C25s came with Android 11 out-of-the-box and it’s anticipated that it will receive an Android 12 update.
Check Out: Realme Android 12 Update list Based on Realme UI 3.0
Hence, the Android 13 update will be the last major OS update on this device.
The Realme C25s came out with Android 11 with out-of-the-box.
Once the update is available on your device, you can head over to Settings >> About Phone >> Software Update section to update your Realme device with the latest security/software update.
For more Realme Android updates please download our Android App – Available on the Play Store