One of the most welcomed aspects of the new Samsung Galaxy S10 phones apart from their cool hardware is the ability to map the Bixby button to a function of their choice. Samsung is now bringing the functionality to older flagships such as the Galaxy Note 8, Galaxy Note 9, Galaxy S8, Galaxy S8 Plus, Galaxy S9, and Galaxy S9 Plus.
After installing the Bixby update that is now available in the Galaxy Store, go to the Advanced Features in the Settings app and then tap on Bixby key. Now here's a catch. You can remap the Bixby button to launch a third party app with either a single or a double tap but not with both. That means, one of the single or double tap gestures will still continue to open Bixby Home depending on the option you select. A long press will take you to Bixby Voice.
Samsung is probably doing this so that users don't abandon Bixby for good but still get some flexibility with assigning the button to something useful instead of letting it sit idle.