Configuration changes in JBossAS are mainly done by editing XML files.
There is a JMX Console (available via http://localhost:8080/jmx-console) but that it mainly a generic JMX browsing mechanism, and while it lets you change the read/write settings, those changes are only temporary.
There is also a web console (http://localhost:8080/web-console) but depending on which version of JBossAS you are using it could be broke - it was working for most of the 4.0.x versions, broke in 4.2.x and the early 5.0 betas, but appears to be working again in 5.0.0.CR2.
In addition, the 5.0.0.GA version is supposed to include a new administration console. This admin console comes from the JBossON product which is a complete monitoring and management solution, but is not free. http://www.jboss.com/products/jbosson