Hello,
You may add this custom class for the button http://prntscr.com/h84etv custom-button
and add this code in Theme Options > Styling > Custom css:
.home .custom-button {
border-radius: 7px !important;
line-height: 24px !important;
font-weight: 600;
}
At first, please read slider documentation https://www.themepunch.com/revslider-doc/slider-revolution-documentation/
Regards