where to modify the mail do not forward rights management policy in Office 365

DNF is a "special" template, you cannot modify it. You can however hide it from Outlook if you want, as detailed here: https://docs.microsoft.com/en-us/azure/information-protection/rms-client/client-admin-guide-customizations#hide-or-show-the-do-not-forward-button-in-outlook

The above is for the "button", if you want to remove all the menu entries as well, use the reg keys detailed in this thread: https://techcommunity.microsoft.com/t5/Azure-Information-Protection/Disable-quot-Do-Not-Forward-quot-RMS-Template-for-Outlook-OWA/td-p/123778