Step 1:

Go to the Products Mapping page from the Dashboard. i.e click 'Map Now' from the dashboard.

Step 2:

Search for the product you wish to change or delete the mapping of for example enter key words and filter by 'Show mapped'

Step 3:

Once you have found the product, click the 'delete product rule' link. 

This will delete the mapping and the product will then be available for re-mapping.