Long-press on the circular icon at the left edge of the first one to delete, then tap the same location on as many others as you like. Each one will change the icon to a check mark. When ready, tap the Delete button at the top of the screen to delete all checked emails at once.
To be honest, however, I find such tasks to be somewhat easier if I do them on my PC. If you have access to a PC, just go to http://gmail.com.