Header cannot be adjusted

Dear all,

My website is http://www.artbasement.nl. I want to make the header narrower and remove/change the logo in the top left corner. Someone else built the basis of the website, and she installed the Header and Footer plugin version 1.4.4 by Stefano Lissa. This plugins’ website does not have a users’ manual or so, and only allows for code to be inserted. I’m not too much of a coder myself, so I wondered if somebody could help me with the code that should be inserted in order to make the header narrower and delete/change the image, or recommend me a more user-friendly plugin that allows for changes in the header and footer.

Thanks in advance!

Sophie