Hello!
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!
You can also redeem your Steam Key from your games library on the Humble Bundle page.