If you would like to use your own google analytics code, you will need to add it in.
However before adding in your own code, you MUST contact support so they can remove the old analytics code. As per Google "Multiple instances of ga.js might result in inaccurate data collection, processing, or reporting. "
How to Add in the Google Analytics Code
Here is a video on how you can accomplish that:
To add in the code, you will need to login to your CRM.
Next click on Websites and then on your domain name.
From the menu on the left handside click on Global Content.
Click on Edit beside GoogleAnalytics.
Click on Source.
Now paste your code.
Click on Save when you are done.