In order to change the nameservers for your domain please do the following:

• Sign in to your Namecheap account
• Click on your username – it will be on the black bar located at the top of the page
• Go to Manage Domains
• Click on the domain you would like to edit
• Click on Transfer DNS to Webhost (or Domain Name Server Setup) to setup the nameservers for your domain
• Select Specify Custom DNS Servers (Your own DNS Servers) option and enter the nameservers accordingly
• Click Save changes

Note that it may take up to 24-48 hours for the changes to take effect worldwide.