how to upgrade from sqeeze to wheezy?

[vykuntam srinivas <vykuntamsrinivas@gmail.com>]

hi,friends.
  i am using sqeeze now and i want to move to wheezy.I 
have googled it and find so many answers,but i  have installed some non free softwares in my SQUEEZE . I think that  it may give  problems  at time of  
upgrade.I am pasting sources.list

> deb-src http://security.debian.org/ squeeze/updates main
>
> deb http://security.debian.org/ squeeze/updates main contrib non-free
> deb http://mozilla.debian.net/ squeeze-backports iceweasel-release
>
> #backports
> deb http://backports.debian.org/debian-backports squeeze-backports main
> deb http://backports.debian.org/debian-backports squeeze-backports main contrib non-free
>
> #mirror
> deb http://ftp.us.debian.org/debian squeeze main contrib non-free
>
> #multimedia
> deb ftp://ftp.deb-multimedia.org squeeze main non-free




can any one please  brief the upgrade process?