I suppose you are talking about migrating from my long ago discontinued dev2day.de repository to the official repository.
The usual update method on Linux, also on the RPi, is discussed here: https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
The migration from dev2day.de to plex.tv (as shown in the video) is discussed here: READ ME FIRST: About Server ARMv7 and ARMv8 Ubuntu / Debian
