You don't need to uninstall unrar-free in order to install unrar-nonfree but ...
# apt-get remove unrar-free
Add non-free repositories eg:
deb http://ftp.debian.org/debian squeeze main non-free contrib deb-src http://ftp.cdebian.org/debian squeeze main non-free contrib
Install unrar-nonfree
# apt-get build-dep unrar-nonfree # mkdir unrar-nonfree # apt-get source unrar-nonfree # apt-get source -b unrar-nonfree # dpkg -i unrar_3.9.10-1_amd64.deb # cd .. # rm -r unrar-nonfree
unrar unrar-nonfree debian
