Godaddy.com:
Login to your Godaddy account and click on your account name in the upper right hand corner. This will bring up a drop down tab and you can then click on Manage Domains:
Click on the domain name you would like to update:
Scroll down to Manage DNS:
After you click on Manage DNS, you will then see the option to change your Nameservers:
Click on the Change button and enter in the new nameservers on the following screen:
After they are entered in as shown above, click Save.
It can take up to 24 hours for the change to take effect.
Namecheap.com:
Login to your Namecheap account and click on Domain List:
Find the domain you would like to update and click the Manage button:
Scroll down to the Nameservers section and choose Custom DNS:
Enter in the new Nameservers as shown below and make sure to click the green check mark to save them:
It can take up to 24 hours for the change to take effect.