Click Erase iPhone.
This erases the device but not any of your previous backups stored in iCloud.
If your device is offline, the remote erase begins the next time it's online. You receive an email when the device is erased.
From the prompt, click Erase.
Enter the Apple ID password and click Next.
If prompted, enter the Two-Factor Authentication code.
Review the Apple ID & Privacy prompt then click Continue.
If prompted, from the 'Trust this browser' screen click 'Don't Trust' or 'Trust'.
If preferred, enter a phone number then click Next.
If preferred, enter a Message in the text box then click Done.
Now that the device is erased, click Remove from Account.