First of all, just in case you are asking What are nameservers? Now that is out of the way.
To make DNS changes at Namecheap.com, log in 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.