Once you complete the last task, select Get certificate on the open certificate screen. The information is pre-filled for you but you can also change it. Select Send certificate to email the certificate to yourself.
I can't find my certificate
If you can't find the certificate in your inbox, check your spam/junk folder, as the email could have landed there. If you want to find a certificate for a previous level, search for "Yousician Certificate" in your email. The certificate is included in the email and you can access it at any time.
If you have lost the email, go to the Learn → Missions tab and select the Certificate button. Check the information on the certificate screen and select Send certificate to email a new copy to yourself.