Google Tag Manager
Google Tag Manager is a free tool that allows you manage and deploy marketing tags (snippets of code or tracking pixels) on your website (or mobile app) without having to modify the code. ... Information from one data source (your website) is shared with another data source (Analytics) through Google Tag Manager
Sign into Google Tag Manager account from here- https://tagmanager.google.com and click on Create Account button.
You only need the code that needs to be added in </head> of your website. Copy the header code (shown inside red circle in below image)
Thats it. Make sure to click on Publish button click Publish button after making the changes.
You can read more about Tag manager on Internet or here -https://www.orbitmedia.com/blog/what-is-google-tag-manager-and-why-use-it/