Even though it's the accepted answer, the previous answer doesn't actually answer the question. While it's true that raspbian isn't as popular as Ubuntu, that doesn't indicate that one shouldn't believe there aren't mirrors. There are in fact plenty of mirrors and having a mirror can save you from having to wait days to successfully update your system.
You can change the source of your apt packages by editing the file
/etc/apt/sources.list . You'll need to use sudo since the file is owned by root. So you might try the following in the terminal...
sudo nano /etc/apt/sources.list
Once you open the file you should see a line starting with
deb. Below that is a
deb-src line for packages to help you compile packages from source, which you can probably ignore. You may or may not currently be using mirrordirector.raspbian.org and that may or may not give you the best results. To try it change the url in the
deb line to
http://mirrordirector.raspbian.org/raspbian/ . If that doesn't work well for you, then you can switch the URL with one that seems nearest to you from this list https://www.raspbian.org/RaspbianMirrors .
There's another file for apt sources in recent raspbian open it with...
sudo nano /etc/apt/sources.list.d/raspi.list
Then change the URL in the probably top line starting with
deb to one of the URLs from the list linked above like
http://mirror.ox.ac.uk/sites/archive.raspbian.org/archive/raspbian if you are near Oxford England or
http://mirrors.ustc.edu.cn/archive.raspberrypi.org/debian/ if you are in China. Then just save the file and update your repository data with...
sudo apt-get update
Now you are ready to go. Good Luck!