Go to Menu (the three dots in the upper-right corner of the screen) → License → I already have a license.

Then tap Restore purchase from Google Play.
Enter the email address you used when purchasing this license, and your personal data.
Activation error
If any error messages appear, try to activate the license using a different network connection.
If you continue to encounter problems, detailed diagnostics are required. Send a written request to our technical support service. Please attach the error screenshot to your request; indicate your Google play order number (GPA-...) and the Google account address (your email@gmail.com) you used to make the purchase.