First of all you will want to login to your WHM account. Details on how to login to your Reseller account would have been sent to you when you first opened a package with us.
1) Select cPanel > Customization
2) Enter your Company Name, this will be displayed on the top left of every cPanel account and Webmail account - Replacing our 'cloudabove' name. Within this customization page you also have access to upload your own company logo should you wish to display your specific branding.
3) Click Save at the bottom of this page and that's it! This is how our Poldice company name would be displayed: