open-gpstracker: Export all tracks or access the SQLite database
Each app on Android stores its data on internal storage by default, below the /data/data/&lt;package_name&gt; directory. Due to the sandbox princip, except the app itself and root nobody has access to that if not explicitly defined otherwise by the developer (e.g. Skype currently has a serious security flaw giving everybody read/write access to all its data, including plain-text passwords). So no, you won't be able to get access to the database, which most likely is located in the /data/data/nl.sogeti.android.gpstracker/databases/ directory without having root access to your device.

As for your second question: You are assuming correctly. As you don't have access to the developers certificate (for good reasons), you cannot "update" the original app with your modified version. You would have to uninstall and then re-install -- or giving your modified version a different package name. In both cases, you won't have access to the original apps data.