This kind of answer doesn’t really make it easy for us to help!
Why can’t you update? What happens? Or better: what does not happen? Error messages? Error logs? Anything?
There are a lot of informations to be found here:
If you have followed the tutorial for a linux installation on a raspberry pi, these commands should update java:
sudo su
echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu xenial main" | tee /etc/apt/sources.list.d/webupd8team-java.list
echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu xenial main" | tee -a /etc/apt/sources.list.d/webupd8team-java.list
apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys EEA14886
apt-get update
apt-get install oracle-java8-installer
exit