Well, I use the delete key on the keyboard. But I work almost exclusively in the list part of the three pane home screen. Other choices would be right click and select delete from the menu. (In V128 it appears as a trash can icon on top of the menu. Or click the button on the email toolbar (it is on the right beside the From: when you are reading the email. It also has a trash can icon.

I would expect you would have had need to delete an email before now having used Thunderbird for years, so what is the question about really? I think there is more than what the question says.