(There are many different free plugins that can do this, however, this one is widely reviewed and has great support.)
First, go to the Plugins tab in your dashboard, and go to add new.
Search for the Head & Footer Code plugin in the plugin search. You will find the corresponding plugin:
After you install and activate, you can find the new plugin under settings > Header and Footer
When you go into this plugins dashboard, you can access different areas by navigating through the tabs. It defaults to the Head and Footer sections to edit. If you are inserting any head tags such as Links or Scripts, you can do it in the Head Injection section, either for every page or home page.