Install Java 15 from APT

apt install openjdk-15-jre-headless

Install Java 15 from source

wget https://download.java.net/openjdk/jdk15/ri/openjdk-15+36_linux-x64_bin.tar.gz
sudo mkdir -p /usr/lib/jvm
sudo tar zxvf openjdk-15+36_linux-x64_bin.tar.gz -C /usr/lib/jvm
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk-15/bin/java" 1
sudo update-alternatives --set java /usr/lib/jvm/jdk-15/bin/java
java -version

Versions and Download Links Click here to see all available versions

OPTIONAL TO SWAP THE VERSION:

sudo update-alternatives --config java