If you purchased a previous iOS or Android version of n-Track Studio the upgrade to v8 is free up to the 31st of January 2017. After that date the app requires an in-app subscription of $0.99/month (Standard edition) or $1.99/month (Pro edition).
To activate n-Track Studio 8 make sure you have the old app installed when you launch the new. The new app detects wether the old version was installed and activates the free subscription up to January 31, 2017. You can remove the old app once the 'Upgrade to the full version' box no longer appears.
If you have already removed the old version please follow these instructions to restore it.
Note that the upgrade is not valid across different operating systems, i.e. if you purchased n-Track for iOS you can upgrade to n-Track Studio 8 for iOS but not for Android, and vice-versa.