From your Dashboard, click on Account, then on My Products.
Find the product you want to update the price formula for. Now, you can update this in two ways.
1. Click on the title of the product to land on its page.
On the top right part of your product’s page, just under Product Groups, you have Price Formula and Returns.
Now, click on Price Formula and choose the one you want for your product.
Click Save and you are all set.
2. From the My Products list, select the product you want to update.
On top of your product list, you can find a field named Assign Price Formula.
Under this field, you can choose to either add the product to a specific price formula or your Default one.
Make your choices and click on Apply.
And this is how you update the price formula for a product. :)