If you want to update your personal DNS manually for testing purposes, you can edit your hosts file to contain a mapping for the IP address of your server and the URL you want. Note that this is a testing solution, not one you should use permanently. Here is a link on how to do something like this:
If your DNS seems to be an issue, it might be your DNS server or it might be how you registered the DNS information. Try changing DNS servers. For instance, Google has provided free DNS servers that you can use (one is 22.214.171.124). There are also free ones out there (like 126.96.36.199, etc.)
To check the contents of a DNS server, open a command prompt and type "nslookup". This will select your default DNS server and then allow you to query it. Enter your URL (just the www.xyz.com portion, not the HTTP:// part). See what IP is returned (if any). This will tell you if your DNS has properly registered your name/IP. You can also switch DNS servers (for example, to try out Google's to see if they have the right info) by typing "server 188.8.131.52" (to switch to the 184.108.40.206 server). Then run the same URL check again to see the results.