How to paste your Meta Tag in your Shopify homepage (theme.liquid)
Login to your Shopify Admin Panel and click on Online Store on the left-hand side menu and select Themes.
Choose Edit Code from Action button drop-down and select the theme.liquid file.
Paste the meta tag that you copied on a blank line directly below the opening <head> tag and and before the first <body> section.
Save your theme.liquid file.
Return to Google Merchant Center. Click on VERIFY URL and then click on CLAIM URL. For more information, go back to the Claim Your Website article.