TuneECU must be downloaded from the TuneECU web site.
If you have an earlier version installed, uninstall it from your device (in Settings/Apps) and delete the users.lic file (if exists) in the TuneECU folder.

Install and open the app, read and accept the License Agreement, read and allow the Privacy Policy.
Allow TuneECU to access to all files on your device.

Choose your account (the account you used on your Android device).