You cannot.
They are part of the hidden server settings.
https://support.plex.tv/articles/201105343-advanced-hidden-server-settings/
and thus can only be changed by editing the Preferences.xml file in the Plex data folder directly.
You need to get access to the Plex data folder first, which is also required for manually obtaining log files etc. You can therefore follow this article: Synology FAQ - Questions, Answers, and How-To's - #19 by ChuckPa
