You can do so by going to the Design page and select the "Pre-order button style" as '"Add to Cart" button style'.
- Go to Design page
- Select “Add to cart” button style in the Pre-order button style section.
Click OK on the pop-up
If you have Pop-up blocker turned on, please allow the pop-up for this site. It is necessary to read button styles and update preview.
Check that the preview of the pre-order button has been updated with appearance of add to cart button.
Note: that if you publish a new shop theme then pre-order button styles remain from the previous theme. Please follow the instructions on how to update here.