As Skyline has alredy told you, the first thing you've to do is to point the nameservers to ns1.x10hosting.com and ns2.x10hosting.com (this usually takes some time to check it)
Then you can do it in two ways:
1) If you want to be able to access www.newdomain.com
but you want to DELETE the greenear.x10hosting.com domain, go to "Account > Modify Account
" and change it there.
2) If you want to access your website by the new domain, and the old subdomain, then park in cPanel the new domain.