When any updates for C3 is rolled out? How do we know about it, and how does it update automatically? Do we need to clear cache?
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No, we've thought of it all. https://editor.construct.net will always load the most recent version, no cache clearing required.
Releases pages auto updated when new release is out as well:
https://www.construct.net/make-games/releases
Does the editor informs the user that is has been updated?
Yep