How do I change the gridmarket Logo to something else?

Hi all,

Despite of all my efforts I could not remove the “gridmarket” name in the header and replace it with the store name.

Also, I uploaded a new header picture in the “header” section of the them and I cannot make it to show up on site.

I thought that using these themes was going to be very easy and self-explanatory but I am getting more and more frustrated at each step.

Thank you!