Update Sony Xperia C5303 ( with Android 4.3 ) to new version? Is it possible upgrade or update Sony Xperia C5303 ( with Android 4.3 )  to Android 8.1 with Xperia Companion software ?
Or any description else ?
A simple and clear NO.
Using the Sony companion app you can only install official updates. Your device is so old that Sony stopped supporting this device long before Android 8 was released.
Keep in mind that Android is not like Windows, every update has to be adapted to your device. Therefore providing updates for a device costs a lot of money which is the reason most manufacturer drop device support very early. Some cheap devices never get a single update.
Therefore the only newer Android releases for your device are unofficial (aka customROM, community builds by skilled Android developers) if they exist. Check xda-developers forum for your device for newer firmware updates. But for installing them you need to root and unlock your device which can go wrong and brick your device.
If you are lucky you may find an Android firmware based on Android 5 or 6. I doubt that there are newer ROMs.