How to edit Document Body/body-text "object" in Business Letterhead Template

Hi,

I am using the Business letterhead, and I want to customize it by removing the address box on the left margin (that was easy to do, just select the text object and hit delete), and then extend the body object back to the left margin. However, I can't figure out how to modify the dimensions of this "object" that is used for the text in the body? Referencing the screenshot below, can anybody help point me in the right direction as to how this kind of "object" is elected, edited, etc.? I am not even sure what terms to search on in help/online to figure out how to use this or how it is being done? Many thanks for the help :-)