Browser is not Updating CSS

Question

I am using CSS in one of my projects, but many a times it doesn't work in expected manner like if I Create a new Class in Css and then updates the Label, Image or anything else's CSS Class to this new class then it doesn't work and when I inspect these elements then i find that they are still using the old class of CSS. Now, if I delete the Cookies and History and then again run the Project then this time it start working.

Can anyone tell me Why it's happening????? And How can I get rid of this problem???????