This post has become obsolete as it was incorporated in the Setting up your Application Server
After decryption of payload with the AppSKey (either by KPN or by the Application Server) the payload still needs to be decoded. All payloads are hexadecimal, meaning they have to be interpreted (decoded) in some way to be used in applications. The decoding scheme depends solely on what the device manufacturer programmed in your device.
For instance, some devices use a simple hex-to-string function where each byte can be translated to a readable characters.
Other devices identify a subset in your payload (for instance the first 4 bytes) and translate that from hexadecimal to decimal to get a sensor value, like the LoRaMote (see here for instance).
Please refer to (a manual from) your manufacturer to get the correct decoding scheme,