First of all it should be understood that the header by default is actually always over the content. If any unnecessary space appears before the content of the page, two options are possible:
1. Most probably there is top padding set for the first row on your page. To remove it:
a) go to Row options:
b) set Top padding to 0px:
2. Top padding for the page content may be set to other than 0px.
If you have updated the theme from version 1.4 or below to version 1.5 or above, you have probably noticed that there is an update available for Revolution Slider.
If you are willing to update Revolution Slider from v.4.x to v.5.x, please read this article on how to do that to avoid any issues after the update.
Do not update using autoupdater built in the theme. While for some of you it may work fine as well (e.g., preview slider demos needed only few changes after the update), some can lose part of your layer or slider settings and will need to set them up again.
Archive with the plugin can be found in tilt / inc / plugins / revslider.zip