The itch.io PC version of the game is no longer being supported by the devs. They provide a Steam key with the game on here, but for those of us who can't use Steam (there's a lot of reasons it might not be an option, including Valve having literally turned the client into a health hazard for some of us), that doesn't help.
If Steam isn't a problem for you, click onto the download page for the game and there's a button to obtain a Steam key. If it is a problem, contact support@itch.io for a refund.
They said they're looking into possibly doing so. They haven't promised it's going to happen confidently, haven't provided a timeframe in which it will happen, and haven't fixed the fact that it currently isn't supported. So for now, the only option is not to use the itch.io version until they fix it.