Hello everyone!
I have OSMC v2019-04-1 installed on a Raspberry Pi 3.
I set it up perfectly on a skin called Phenomenal, it took me weeks to get every setting and personalization option to fit my needs.
From one day to the other OSMC autoupdated itself to the latest version in which the Phenomenal skin isn’t supported anymore, changing it to the default Kodi interface, destroying weeks of work I put on the previous skin e pretty much changing every single little setting I had.
Is there any way to prevent OSMC or the Raspberry Pi from upgrading?
Thanks in advance for your answers!
OSMC will never update itself without your consent. You must have accepted a prompt to install updates.
Update checks can be configured via My OSMC.
That’s really weird then, cause from one day to the other a little icon with the OSMC logo on it appeared at the bottom left corner and a text “New update” (or something similar).
The logo couldn’t be selected or clicked on. It stayed there for the entire day and after a reboot the whole upgrading process began. 
Actually as @sam_nazarko wrote it wouldn’t update without consence but anyhow he also showed you how you can disable the download of the update all together.
Also it’s important to know that most likely only major Kodi changes like from 17 to 18 would normally give issues to skins/addons. Such updates normally only happen any 1-2 years
I will definetely check the option that @sam_nazarko mentioned
Thanks for the help!