This differs from course to course. Sometimes you qualify for a scholarship because your university has an agreement with Utrecht University or HU University of Applied Sciences, sometimes a foundation sponsors specific target groups, and in other cases the course leader gives a waiver to a few outstanding students. If available, scholarships are clearly indicated on the course pages, under "scholarships".
In case Utrecht Summer School does not offer scholarships we advise you to contact your home university or local institutions for financial support. You can also have a look here for more information about scholarships.