3.0.1 is the latest stable release of Glyphs. 3.0.2 is currently a “cutting edge” version, which may not be as stable as 3.0.1, but it already includes the latest fixes.
You can wait for 3.0.2 to be released as a stable release, which may happen anytime between now and in a few weeks, depends on what the Glyphs developers are currently working on.
Or, you can opt into using the cutting edge release by checking the checkbox in Menu → “Glyphs” → “Preferences…” → “Updates” → “Show cutting edge versions”. Click “Check Now” to search for and download the latest cutting edge version.
The cutting edge version is typically quite stable (I use it daily), but if you want to avoid using the unpolished cutting edge version there is also a workaround for your specific problem.
Go to the “Features” tab in your font’s info window and add the ‘kern’ feature. Once added, select it in the sidebar, press Return ↵, rename it to ‘case’ and press Return ↵ again to confirm. Glyphs may show an error dialog, but you can ignore that. Now you can edit your ‘case’ feature code, no cutting edge version needed.