Hi everyone!
First of all, I’m rather new to Grav CMS, and I really like it. Thanks.
I was reading in this archived post here, that “There is no built-in support for updating private themes or plugins using Admin, at the moment.”
Since this post is from 2017: Is there any update on this regarding a built-in support for updating private plugins? Or is the best way to use the Direct Install Tool in the Admin Panel?
Best regards and thanks.
AFAIK, nothing’s changed in that regard. Besides direct install, if your plugin is public, you could also submit it to GPM (sadly, can’t find the requirements and the process now 
Maybe @anon76427325 has a link). If approved, it would become available via Admin panel
@jana, Like, Karmalakas, I’m not aware of any changes with respect to installing/updating private themes/plugins. You could use either Direct Install from Admin, or the cli ($ bin/gpm direct-install ...), or custom shell scripts of course.
If you do wish to share a theme/plugin with the community, topic Theme/Plugin Release Process from the docs might be handy.
Before submitting a theme/plugin, asking the community for a review of the theme/plugin might be valuable. Grav’s devteam does not perform reviews before adding the theme/plugin to the GPM.
@Karmalakas @anon76427325 thanks a lot for your fast replies, I appreciate it!