Cange page width

I know this has been asked on several occasions, but I cannot figure out how to change my page width. It was fine until I updated to 3.9, but now it is not full width. My site is https://www.catholictravelblog.com and I am using a child theme of 2012. Not too familiar with css, so I am afraid to change anything unless I now for sure that it is the right thing to do.