1. go into the mp_Sites table and change IsServerAdminSite to true for site 2 and false for site 1.
2. add this in user.config if you don't already have it:
<add key="AllowDeletingChildSites" value="false" />
3. Touch web.config to clear the cache
4. Delete site 1 from the UI
Now site 2 is the default site
Hope that helps,