Microsoft has updated Cortana for Android, adds ability to replace Google Now
In the latest Beta version of Cortana, you can now replace the Google Now shortcut, which can be accessed by pressing and holding the home button. Instead of Google Now, you can activate Cortana instead (see image below). This build of Cortana has yet to support “Hey Cortana” and there are still other ways to access Google Now features if you really need it.
For anyone interested in testing Cortana on Android, there are several places where the Beta .APK file can be downloaded. For anyone looking to be a part of the official Microsoft Beta program for Cortana on Android, more information