How can change the default URL to access my application ?

The default Apache document root directory is "/installdir/apache2/htdocs" folder. You can change the document root in the "/installdir/apache2/conf/httpd.conf" file to run your application in the root URL.

If you want to change the default URL from "http://example.com/your_application" to "http://example.com", you should modify the "DocumentRoot" and "Directory" lines in the httpd.conf file to point to the application "htdocs" folder. It is also necessary to comment the "Include" option. You should replace it "your_application" with drupal, wordpress, joomla or your own application.

httpd.conf

...
#                                                                                                     
# DocumentRoot: The directory out of which you will serve your                                        
# documents. By default, all requests are taken from this directory, but                              
# symbolic links and aliases may be used to point to other locations.                                 
#                                                                                                     
DocumentRoot "/installdir/apache2/htdocs"
...
#                                                                                                     
# This should be changed to whatever you set DocumentRoot to.                                         
#                                                                                                     
<Directory "/installdir/apache2/htdocs">
...
#Include /installdir/apps/your_application/conf/your_application.conf
...

Some applications also require changes in their configuration files or in the database. For instance Wordpress also saves the URL in the database. If you have changed the URL or you are trying to access from a different machine and you can see Wordpress without CSS, you have to modify the domain name or the URL you should run the following command to modify the database:

$ mysql -u root -p -e "USE bitnami_wordpress; UPDATE wp_options SET option_value='http://example.com/' WHERE option_name='siteurl' OR option_name='home';"

You can find more information in the Wiki page for each project.