when I open my browser to go to PMS, there is a message: “An update is available. Please install manually”.
how do I install an update manually on a Raspberry Pi ??
please not, that it is running headless.
Same as you installed it in the first place through SSH. If you have it pointed to the repository I would assume apt-get would work, otherwise you’d have to dpkg -i the file that works for Raspberry. Do also note that a Plex update on a Linux based system will not touch your libraries, settings or such so it is safe.
I have installed PMS through the repository.
and updated it once or twice with apt-get.
but now this does not work.
or do I have to wait a few days for the update to appear in apt-get ??
If I remember correctly on how to set it up on a Pi, it is most likely pointed to a repository not hosted by Plex themself but a dedicated forum member who maintains it on his sparetime - so yes, it will most likely take a day or two 
the repository has been adjusted and the update has been installed through apt-get.
