How to change the fixed 960 px width to 1260?

In order to set your fixed with site to 1260px rather than the default 960px you would need to define a new grid. You can read more about that here: http://fusiondrupalthemes.com/support/theme-developers/grid-concepts/cre...

Another approach would be to just enable the fluid layout on your theme's configuration page and use that. It goes to 1320px max-width and scales down to 768px. Not sure if that is an option for you but hopefully you were aware of that choice :)

Hope that helps!