Revising a Template so that it looks good on a desktop and on a mobile phone

We just paid a freelance person to build us a Constant Contact template. It is built with two columns and looks great on a desktop. But, the client is asking for space to be removed from ONE side of the template when it is viewed on a mobile phone. My boss said the change can be made to the code on the one side and not affect the other side. He also said the side that doesn’t need space removed will not be affected if the template is edited using code. Does anyone know how to edit a template so it will look good on a desktop and on a mobile phone? Thank you, Theresa