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Update for SharePoint 2013:

I recently set up a SharePoint 2013 site and provisioned the MySite host as "My.domain.com" when I should’ve used something like "MyTest.domain.com."

In order to fix this, it’s very similar to fixing it in 2007. 

Change the alternate access mapping.

Edit the default zone URL.

Edit the Site Binding in IIS to reflect the new host header.

Manage Service Applications

Click on your User Profile Service Application to manage it.

Go to Setup My Sites.

Change the My Site Host location.

IISReset and you should be good to go.

Hey, and if you want to change the name of the web application so it matches (I know, I know…it’s a bit anal, but I’m kinda’ OCD), here’s some PowerShell.

$WebApp = SPWebApplication | where {$_.Name -match "OldWebAppNameGoesHere"}
$WebApp.Name ="NewWebAppNameGoesHere"
$WebApp.Update()

Original post for SharePoint 2007:

I was playing around with SharePoint on my virtual machine and had set up a new installation of MOSS when I noticed I had entered a host header incorrectly (at least incorrectly from the standpoint of being consistent with my previous installations). I entered "MySite" instead of "MySites" as the first part of the host header. I know, I know, it’s probably more correct to use the singular, but I was trying to be consistent with what others had done.
All I wanted to do was to change the host header and have SharePoint recognize the change. I performed the standard internet search and didn’t get instant gratification, so I started playing around. I found that making the changes in three places and an IISRESET seemed to take care of my problem.
First, in Central Administration under Operations, Global Configuration, Alternate access mappings, edit the internal URL.


Next, open up IIS Manager and select the properties of the MySites web app. Change the host header by clicking on the Advanced… button.


Again in Central Administration, navigate to the Shared Services provider and click on My Site settings in the User Profiles and My Sites section. Change the personal site provider URL.


Perform an IISRESET. All should be well.