If you have access to the database, you'll want to add a record for the new URL. There is another way to "spoof" this though, so that it appears to work for you, that doesn't involve DNN.
If this is on one machine... simply spoof your network routing by making a change to your "hosts" file which you can find here: c:\windows\system32\drivers\etc\hosts ( note that it has no extension ).
Here is the default format of what you'll find in that file:
# Copyright (c) 1993-1999 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host
127.0.0.1 localhost
But you can add any additional records like this:
127.0.0.1 www.anydomain.com
So that your local machine will now route that domain request to 127.0.0.1, which is your root web. This works very well for testing your domain setups.