The JDK used by the Application Server processes for all the domains in a given server installation is determined by the appserver-installation-dir/config/asenv.conf file. The property AS_JAVA in this file determines the JDK used and is set at the time of installation. If a different JDK is to be used by Application Server processes after the installation is completed, this value can be modified to point to another JDK. Note that all domains in this installation will be affected by this change.
Manual changes to asenv.conf file are not checked for validity and hence care should be exercised while changing them. Check the product documentation for minimum JDK version requirements when modifying the value for AS_JAVA.