Rename sharepoint URL

we are using in our sharepoint 2013 as our server name, called "sharepoint"

This is runs only in intranet. URL : http://sharepoint/

Now we have to change the URL : http://sharepoint in to http://sharepointportal/

we tried in central administration alternate access mapping, but it is not working.

we would like to change only URL, not in servername

How to solve this issue?

Jey