Select the table
In the ribbon go to Table Design
In Table Styles, find the selected style, right click and Duplicate
In Table Styles right click the bottom down arrow to open the entire pallet of styles
Under Custom in the top row is the duplicate you just created. Click it to apply it to your table
Right click on that style
Click Modify
In the listbox click Total Row
Click the Clear button under the list to clear the formatting.
Click OK
Your total row should now be not bold.