To make DNS changes at Namecheap.com, login to your account and follow this guide.
1. Select the domain you wish to change and click 'Transfer DNS to Webhost'
2. Enter the new nameservers in the 1st and 2nd fields, click 'Save Changes'
Note: You need to enter a minimum of 2 nameservers.
Warning: Do not enter your IP address, this will cause problems.
Remember it can take up to 72 hours for the changes to take effect.