The Cortana beta available for Android users has been updated by Microsoft. The company has added the feature to enable users of the assistant to set Cortana as default, replacing the pre-loaded Google Now. This alteration will also enable you to press and hold the home button to activate the personal helper.
Unfortunately, there's not yet a way to bring up Cortana via voice command, like "Hey Cortana". But it is possible to continue enjoying Google Now services should you require them, whilst Cortana is configured as your default assistant.