Hello aurasalunen!
There is no need to get a Steam account if you got the game from Humble.
If you directly downloaded the game from Humble please follow these steps to update:
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!