You can see in our Demo store that Shopify is not showing “Free” delivery text anymore. Instead of “Free”, it is showing “1-3 days” text.
There is a simple way how to achieve it.
Go to your store admin -> Online Store and from Actions choose Edit languages (You can Duplicate theme before if you would like to). Move to “Checkout & System” in Edit languages view and search for “Free rate label”. Change “Free rate label” to your desired text, just keep in mind that this text will be shown to all free delivery methods.