Use the mirrors listed Here and download the 4.8.1. The process is pretty straightforward. I would recommend to use this Procedure to complete your installation.

As you may know GCC doesn't support "make uninstall" and it has been suggested that you install GCC into a directory of its own and simply remove that directory when you do not need that specific version of GCC any longer. Hope this helped. Cheers

Edited: The Option 2:

I assume that you already have a former version of gcc, the easiest way could be adding PPA to your repositories and Update and upgrade you can have the latest version with no worries:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test

sudo apt-get update

this will add the new PPA to the other sources.

Then unistall the alternative:

sudo update-alternatives --remove-all gcc 

sudo update-alternatives --remove-all g++

then:

sudo apt-get install gcc-4.8

sudo apt-get install g++-4.8

and as the alternative packages install :

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 20

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 20

sudo update-alternatives --config gcc

sudo update-alternatives --config g++

at the end:

sudo apt-get update

sudo apt-get upgrade -y

sudo apt-get dist-upgrade

Hope this changes the --version ;)