First, go into your Dashboard, click on Account, then My Products.

Now find the product you want to update the board(s) for. 

You can go on in two ways:

1. Click on the title of the product to land on its page. 

On the top right corner of your product page, just under Product Status, you have Product Groups. 

Under Product Groups, you will find Boards. Just click on Board(s) and make your changes. 

Then click Save and you are all set. 

2. From the products list, select the product(s) you want to update.

On top of your product list, you can find the tab Update Boards. Now, you have to choose either to add the product to boards or remove it from boards. 

After you made your choice you need to click on Apply and that's all.

And this is how you update the boards for a product.