We are going to go to edit theme code and find the product-template.liquid file in the sections theme.
On line 239 you will see the div class product-single__description. See image below. [cid:image001.png@01D5E30A.3438E660] We are going to move the div and place it on line 245 before the last closing div. [cid:image002.png@01D5E30A.95290810] That makes the description full width.