In WHM, you can change the default PHP version through Multi-PHP Manager.
However, if you want to change the global default PHP version through command line you can follow below steps :
To display the current default PHP version :
# /usr/local/cpanel/bin/rebuild_phpconf –current
Above command is showing PHP 5.6 as default
# /usr/local/cpanel/bin/rebuild_phpconf –default=ea-php70