1. Install the Insert Headers and Footers plugin.
2. Go to Settings > Header and Footer Scripts.
3. Paste the code snippet into Scripts in the header textbox.
4. Click Save Settings.

Alternative Method

1. Navigate to Appearance > Theme Editor
2. Click the Theme Header (header.php) file on the right-hand side below the Theme Files section.
3. Paste the code snippet before the closing </head> tag.
4. Click Update File