#install new versions of gcc and g++
sudo apt-get install gcc-5 gcc g++-5 g++

#remove existing alternatives
sudo update-alternatives --remove-all gcc
sudo update-alternatives --remove-all g++

#add new and old version to update-alternatives db
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 50 --slave /usr/bin/g++ g++ /usr/bin/g++-5
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-6

#choose the needed version
sudo update-alternatives --config gcc