This is a slightly broad question, so the answer will be as well. I suggest you ask additional, specific, questions as you continue your process.
First steps would probably be to get the apk, extract the java & native code from it and reverse your way to the encryption algorithm. Once you figure out what's the encryption algorithm and protocol you can see if the application can technically decrypt data it previously encrypted.
There are already questions here on how to start reverse engineering Android APKs.
Once you have the java code decompiled, you can search for that code that actually performs the encryption and see what are the functions it calls and the input it receives.
This is not necessarily possible because some encryption models (such as private/public encryption) only allow one side to decrypt.