If you're not a fan of the Bixby voice assistant, then you can follow the instructions in this post and you will learn how to do it.
You cannot disable the Bixby button completely and apply it a different function, but you can at least prevent it from launching Bixby when pressed. Most Galaxy S9 users prefer the Google Assistant functions to Samsung's Bixby one.
The first thing that you need to do would be to login into your Samsung account, as otherwise you might not be able allowed to tinker with the phone's settings. Add your Samsung Account and then we ca continue to the next tutorial.
Disable Bixby on Galaxy S9:
Go to your phone's home screen and tap + hold on the empty space to load all the "home screens". Find the "Bixby" screen and disable its toggle to stop Bixby Home from launching.
Now you will have to disable Bixby button and prevent it from being launched on your Galaxy S9: go to Settings> Advanced Features and here you have to find the Bixby menu - open it and then disable the Bixby button.
These were all the steps that you had to take in order to disable the Bixby feature on your Galaxy S9. If you need more help completing this guide, then you should tell us in comments below.
In case you're having trouble launching Bixby and you found this guide by mistake, then you should simply use the same instructions as above, but instead of disabling the feature you have to use the enable function.