1. Check the current version out of curiosity

> /usr/libexec/java_home -V

Expected result should be similar to

Matching Java Virtual Machines (2):
    9.0.4, x86_64:      "Java SE 9.0.4" /Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents/Home
    1.8.0_162, x86_64:  "Java SE 8"     /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home

/Library/Java/JavaVirtualMachines/jdk-9.0.4.jdk/Contents/Home


2. Update your .bash_profile with shortcuts to swith between versions easily:
from whatever you are, go to you home dir
> cd
> vi .bash_profile

add lines like:
alias j9="export JAVA_HOME=`/usr/libexec/java_home -v 9`; java -version"
alias j8="export JAVA_HOME=`/usr/libexec/java_home -v 1.8`; java -version"

save and exit (in vi it's `:wq`)
update your bash_profile with

> source .bash_profile


3. You are ready to go.