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In the program window, open the application menu (the button with the three dots in the top-right corner of the screen); select License, then Enter new serial number, and enter a valid serial number. If you purchased the license via Google Play, after selecting I already have a license, select Renew the license from Google Play. To activate the license, you must have Internet access and use the same Google account you used to make the purchase.
You can find more information about all the activation methods at: https://download.geo.drweb.com/pub/drweb/android/pro/HTML/en/index.html?dw_use_license_key_file_new.htm