Hadoop 3.1.2 is the latest version, you can download it with the help of this link https://www.apache.org/dyn/closer.cgi/hadoop/common/hadoop-3.1.2/hadoop-3.1.2.tar.gz
To know the steps required how to install Hadoop, you can follow this link https://www.edureka.co/community/38730/how-to-install-hadoop-in-ubuntu?show=38730#q38730.
And for more query regarding the latest version of Hadoop, try checking this website http://hadoop.apache.org/releases.html. Here you will find the latest Hadoop release. Make sure you use the JAVA version according to the version of Hadoop you will be using.