Hi Elizabeth-
The reason that your logo or site title remains on the left side is because there are actually two columns in the Grid One theme’s header. By default, the social media menu is displayed in the right-hand column.
You can accomplish this with a little bit of custom JavaScript and CSS however. Navigate to Customize > Advanced > Custom JS & CSS and paste this code into your Custom Theme CSS:
.site-branding {margin:auto;}
.site-title {text-align:center;}
Then, paste this code into your Custom Theme JS:
jQuery("div.col-md-6.header-7").remove();
jQuery("div.col-md-6.header-6").removeClass("col-md-6").addClass("col-md-12");
This code will remove the column on the right (header-7) and add the full-width class (col-md-12) to your logo column. The CSS will take care of the rest, and center your logo or site title.