Hello!
If you're playing Slime Rancher through Steam, the game should automatically update to the latest version. You may need to restart Steam to trigger the download of a recent update.
If you directly downloaded the game from Humble please follow these steps:
1) Return to your Humble download page. If you don’t remember your download page please use
https://www.humblebundle.com/resender to have it re-sent to you.
2) Download the new copy of Slime Rancher.
3) Run the installer the same way as when you originally installed Slime Rancher. It will overwrite the old install. All your save games and preferences are stored separately, so they are safe.
4) You’re done! Happy ranching!