Editing a header

Hi
I have uploaded a fishing theme which i like, but i want to edit the header. I am very new to this, so noddy instructions would be great. If you look at my site – https://worldoffishing.org/ on the left hand side of the header there is an oval with the words Fishing Charters, i want to change that to say World of Fishing. If anyone could tell me how to do that i would be very grateful. Many thanks in advance.