You delete cookies and cached files in your browser settings. Your browser is the program you use to browse the internet (for instance Firefox, Google Chrome or Safari).
The approach varies a bit, depending on the browser. An easy way to find out the approach for your browser is to find a guide on Youtube. In the search bar, write: ”delete cookies [name of your browser]”
Here you can see a video guide (Danish browser settings) on how to delete cookies and cached files
in Google Chrome: http://screencast.com/t/GWuXdP2X
and in Mozilla Firefox: http://screencast.com/t/N0rPFggxE2df