Well, it depends. If you have a website you never update, then you don't need to change the copyright year. If you regularly update the content on your website from year to year, then you should have a range of years, with the first year being the year of first publication:
- Copyright 2011 (haven't updated anything since first published in 2011)
- Copyright 2012-2015 (updated content every year)
- Copyright 2010, 2013-15 (didn't update anything in 2011 or 2012)
You don't necessarily need to put a copyright notice on your website, but it does serve as a reminder to the general public that copyright does exist and that you are prepared to enforce it should you need to. You own the copyright to your publications whether you formally register it or not (common law copyright).