Log in to your account with your current domain registrar/Rackzar.
Find the domain name that you want to update and click on its name to open its settings.
Look for the option to "Manage Nameservers" or "Edit DNS" and click on it.
Find the section that allows you to change the name servers. These may be called "Use Custom Nameservers," "DNS Servers," or something similar.
Enter the new name servers that you want to use for your domain. You will typically need to enter at least two name servers for redundancy, and they should be provided to you by your web hosting provider.
Save the changes to the name servers.
It's important to note that changes to name servers can take up to 24 - 48 hours to reflect across the internet. During this time, your website and email may not function properly. If you are switching to a new web hosting provider, it's a good idea to make sure that your website and email are still working properly after the name server changes have propagated.