Hello Dew!
That sounds like you probably got the game from Humble. In order to update the standalone client you got from them, do the following:
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.
That should do the trick.