Thank you very much @WillT for your insightful reply! That is exactly what I was searching for, specially this one https://github.com/PyFR/PyFR/pull/195 High-Order VTK Output Mode.
If I am not a bother, can you enlighten us how to update pyfr? so we can input --order p-order in the export command. Right now in my version, it is not recognizing the command order.