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Open the Dr.Web for Android application, and tap Menu (the three dots in the upper-right corner of the screen) → License → Renew license via Google Play. Open the link and pay for the license. When an attempt is made to renew a license that was not purchased on Google Play, the application will display an error message and another renewal method can then be selected.
You can restore your serial number using the web form at https://support.drweb.ru/restore.
If you know your previous email address, you can replace it with your new one using the web form at https://products.drweb.ru/register/change_email/.