Download the Java SDK (not the JRE) and install it on your system, if you have not already done so.
The Java SDK can be downloaded from http://java.sun.com/j2se.
Completely stop the Application Server.
You can use the following command line:
Alternatively, you can use the Administration Console GUI:
Edit the install_dir/config/asenv.conf file (asenv.bat on Windows), changing the value for AS_JAVA to point to the new J2SE home directory.
Edit the as-install/samples/common.properties file, changing the line beginning com.sun.aas.javaRoot... to reference the new J2SE home directory.
Restart the Application Server.