You should always keep a keystore of his. "Apk" because without it is impossible to make updates of your application that is in store.
A keystore is a unique key per application, you can only update a new version with the same key.
If you lost your keystore you will have to send to the store as a new application.
You can use the Apktool to generate the key for your applications.
Where is this keystore held on my PC?
Look this file
at windows look at your user folder then .rhomobile/keystore