It's not clear to me why port 8088 would conflict with port 81 or port 7071. Nonetheless, to change the port on which the Asterisk-GUI web interface listens, edit /etc/asterisk/http.conf and add (or uncomment) a line in the "general" section. In this example, I've changed the port from the default of 8088 to 8488.
Restart Asterisk for the changes to take effect. If you're using AsteriskNOW, you can use "sudo service asterisk restart" at the linux command line.