Sadly, you can't. I assume you're using the desktop app.
It can only download games through Itch's own platform. As far as I've been able to figure out the past year or so, it simply can't handle third-party downloads. Sadly, as VNs tend to be very graphics/video have, a lot of them just go over the limit Itch can handle.
All you can do is download from a regular browser and run the game directly from its folder, not using itch as a launcher.
That certianly appears to be how it works. Although, a lot of people don't seem to grasp it so maybe it's not universally obvious.
Or maybe it's just more obvious if you follow the Patreon, which people following the free releases likely won't be.
It certainly looks like the current release series is 0.3.x(a/b/etc.)
With the free/itch version always being at .x-1. But, I assume, the most complete iteration of it.
So those of us paying on the Patreon get 0.3.4, which has had three sub-releases so far. But, I assume, once 0.3.5 starts to come out on Pateron, the itch.io version will then be 0.3.4c
But it will "appear" like the Patreon version is the only one getting obvious movement. However, only because the "latest" version seems to get released in smaller chunks. The itch.io version will make fewer, but larger, jumps, to the latest of the previous release.
