Step 0: Select an option from "Behavior of the 'Address Line 2' field" field on the dashboard, and save your settings, after that, the only thing you need to do is to change the label of the Address line 2 field. Easy to set, here are the steps:
Step 1 - Go to Shopify Admin » Online Store » Themes
Step 2 - Click "Edit default themes content"
Step 3 - Search for "address2" » Edit the labels as you want » Save settings
Done! The checkout page has been updated. "Address line 2" is now labeled as "VAT" as you specified. Since this option is enabled in the dashboard, any data entered in this field by the customer will appear in the VAT section of the generated document.
