There are a lot of Q&As out there on how to do this... Some good info here: http://askubuntu.com/questions/159575/how-do-i-make-java-default-to-a-manually-installed-jre-jdk
I usually install my JDK under /usr/lib/jvm alongside the others:
cd /usr/lib/jvm
sudo tar xf ~/Downloads/jdk-8u91-linux-x64.tar.gz
sudo chown -R root:root jdk1.8.0_91
sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.8.0_91/bin/java 1