Google camera version 3.1.021 just showed up on APK Mirror. This update brings the new UI from the 6P and 5X to all other Marshmallow phones. However you won’t be getting the burst mode gif option or slow motion. It should be slowly rolling out from the Play Store, but if you just can’t wait. Grab it from APK Mirror and install it manually.

Google Camera 3.1.021 (2428808-30) (31021130) (ARM)

Google Camera 3.1.021 (2428808-40) (31021140) (ARM 64)

Source: Android Police