In the list of notes ("All Notes" or an opened folder), touch and hold the note you want to delete. The list switches to selection mode. Check the notes you want to delete, tap the trash icon at the top of the screen, and choose "Delete".
You can also open the note and tap "⋮" at the top right → "Delete".
Deleted notes move to the "Trash" and can be restored within 60 days (see "How do I restore a deleted note?").