Begin by selecting the cabinet that you want to change the countertop material of.

Under "Cabinet" Menu, look for the section labeled "Countertop" on the right of the design panel. It will be available both in the "Quick Access" and "Customise" tabs.

Click on the drop down next to it to open the details of the countertop. You will see the "Materials" section there.

The material that is currently added in your cabinet will show there and next it to you can see the "Fetch from Store" option. Select the same and the carousal showing the available countertop options in your catalogue will be generated.

Select any material by clicking on the same.