Remove "GRAV" form the header

I am newbie, I want to replace the word “GRAV” from the header of the pages, how can I do? Can I replace a logo?

You mean in the admin panel? If so, you can modify this by clicking on Admin in the Plugins panel.

If you mean in the frontend, on the other hand, it depends on the theme. I suppose you use Antimatter. In this case, the logo can be changed in the Site configuration, in “Site title”