On Desktop
At the startup screen, click I have an account and then enter your Recovery Password.
Enter your recovery password into the text box, and select a new display name.
Your Account ID is recovered.
On Mobile
At the startup screen, tap I have an account.
Enter your recovery password into the text box.
Enter a new display name and tap Continue.
Select your preferred push notification setting and tap Continue.
Your Account ID is recovered.
To learn about account recovery, check out this video: