To edit existing Product Fields in your Template open the Product Template view by clicking "Products" on the left menu and then "Product Template" on the upper right.
To edit a Product Field just click on it
Change existing field from Product Field to Option Field:
If the created field is an option you can't just set it as Product Field. An option is a field that defines your variants. e.g. "Color" or "Size" is an option field.
To change a field to an option field just tick "The field is an option field and used to create product variants"
E.g. "Color" will then appear in the Product Editor under Options and must be filled out in the table view. See step 4.