Are you looking to change the point where the layout switches from Desktop to Tablet? This is controlled through media queries in the css.
In the CSS, you'll find the following media queries:
@media screen and (max-width: 43.68em) -> small pones
@media screen and (min-width: 43.69em) and (max-width: 59.49em) -> tablet in portrait
@media screen and (min-width: 59.5em) and (max-width: 80.99em) -> tablet in landscape
@media screen and (min-width: 81em) -> desktop
To change the so called 'break point', find and replace all occurrences of the relevant media query and change it however you want. Just make sure that if you change the width, you will also have to update the other queries to make sure there is no gap.
For example, say that you want to change the desktop view to be available from 1025px upwards (as 1024px width is an ipad in landscape). The current desktop is 1296px or wider (81 x 16).
You first count the EM measure:
1025 / 16 = 64.0625
Lets make that 64em.
You change the desktop:
@media screen and (min-width: 64em)
But now you have an overlap between tablet landscape and desktop because landscape goes up to 80.99. So now you change tablet desktop as follows:
@media screen and (min-width: 59.5em) and (max-width: 63.99em)
In RoboHelp 2015 and up, this setting is included in the RoboHelp GUI, so things will get better. For now, you're limted to doing it through the CSS.