Hi. I don't think you can, but I'm not certain. You could change it in the database manually, but I do not know if that would cause problems. I would leave the default alias alone and just add whatever alias you need in addition to the default. You can assign a different domain name to a portal just by adding a new alias and that is very easy to do under Host->Portals. All you need to do is set up IIS to point the domain or subdomain to the root portal using the default alias. DotNetNuke will handle the rest by directing the browser to the proper portal and with the proper domain name in the browser window. Is that what you are trying to accomplish? For example, you've installed DNN at www.123.com but you want web surfers to type in the url www.abc.com to get to your website and have www.abc.com be the url always used and seen. When I did this, I left the default alias there and just added the alias www.abc.com and it was assigned the portal www.123.com/portals/2/. Maybe that's not necessary in your situation, but I still wanted people to access 123.com because it was a personal site and abc.com was a business site and I wanted them treated as two separate portals.
If that is not what you are doing, if you simply want to change 123.com into abc.com, then you should be able to change the IIS path setting at your web host for abc.com to point to the root. Then I would add the alias as a new alias under Host->Portals. Then make sure it was directing properly and working fine. Then you should be able to delete the 123.com path setting in IIS since the abc.com path setting is now the one being used. Now, I don't think there is a way to delete the default portal setting withing DNN, you could probably delete it manually in the database via enterprise manager for sql. But, again, I have not done that and do not know what problems it would cause doing so, so personally, if the abc.com alias was working, I'd just leave the 123.com alias there and ignore it and possibly add a robots.txt file telling the spiders not to index pages on 123.com, that way no one would ever stumble across the old alias in search engines, and when search engines hit the pages on the old alias they should get removed from indexes, too, instead of just not found. Of course, if you want to redirect everyone from the old pages to the old pages under the new alias, well, then that's a whole 'nother solution altogether.
On one of my clients websites we had her pages in the root directory of the site and used Expression Web to create and publish. We then installed DNN in a subdirectory of the root and once we got the dnn site running the way we wanted, we then went into IIS on the web host and changed her domain name to point to the dnn directory. The root directory is now ignored. We did then take her old files, added redirects to each to point to the new dnn corresponding page, and place those old files with redirects into the dnn directory, that way anyone trying to visit the old page from an outdated search engine listing would end up on the proper page, and search engines would get the 301 moved permanently message and update their records automatically when the website was crawled.
There are probably other scenarios and situations that would have again different solutions to similar but very different reasons that all would have reason for changing a websites alias or domain name within dnn. More details about what reasons why you are wanting to change the default would be needed to get the answer you are looking for.