To edit a post, just sign into your Xanga account.  You should end up here:

• http://www.xanga.com/private/home.aspx

Under each post, there should be a link that says "edit it".  Just click on that link, and an editing interface will appear.  Just edit your post, and then click "Save Changes" to re-publish it to your Xanga site.
 
NOTE: If you don't see the "edit it" link, please see:

I put some bad HTML or JavaScript into an old post, and now I can't edit it?!
http://help.xanga.com/canteditentry.htm