I purchased Slime Rancher from your site and can't update

If you recall purchasing Slime Rancher using a "Buy Direct" option available on a prior version of the http://slimerancher.com website, then you likely purchased it through the merchant Humble.

To update to the latest version, all you need to do is grab the latest installer and run it. You can get this by using Humble's Resender tool. You must use the same email address you used when making the original purchase