https://www.theverge.com/2018/5/7/17328758/amazon-alexa-default-voice-assistant-android

Amazon’s Alexa can now be set as your default personal assistant on Android phones and accessed through holding down the home button, as a user on Reddit pointed out. Previously, you were only able to swap in Microsoft’s Cortana as the default voice assistant on Android devices.

Setting up Alexa on Android is going to vary depending on which device you have but the general gist is that you update the Alexa app, then head into your phone’s settings under the category for “Assist apps” and change your selection.