Below is an easy to follow tutorial on how to create and terminate an account in WHM. For those who "just-in-case" need to know how certain things work and also those who are new to using WHM.
On the left hand panel, scroll down until you find the category "Account Functions". Click on the very first link "Create Account".
You will see two columns, the left one for the information on the account you are going to create and on the right information on your hosting space and the packages you created. On the left write in the information for the domain name, a username will be created automatically for you with the first 8 letters in the domain name. You can either go wit that or create your own.
For the password you can either create your own or have WHM generate one for you. This is a great idea for coming up with a random password to use, especially if you are setting up a client account.
Once you create your desired packages/plans you'll notice that they will show up on the right hand side where your total disk space and bandwidth stats are shown. You'll also notice that there are "limits" to how many accounts can be created with the specified disk space/bandwidth packages you made. It'll be a good idea to follow what the limits say and see if one package can only have one account, that you should probably decrease the size. (leaving it would be called overselling)
Once you are done with creating the details, click on the create button at the bottom. You'll then see a page with all the details about what WHM created for this new account and green check marks on the right hand side of each. Congratz, you made your first account!
Upgrading/Downgrading an Account
On the right hand menu, click on "Upgrade/Downgrade an Account" link under "Account Functions".
You'll be taken to the page where a list of accounts on your server are. Once you click them, information on that account will show up on the right side. click on the account domain name that you'd like to upgrade or downgrade.
Before upgrading the account, be sure to check out your account limit stats on the right hand side. This can help you view what your limits are at before upgrading to a package that is already to full.
In the box, you'll notice a list of all of your current packages. If they are highlighted in Green it means it is OK to upgrade or downgrade to that package, if they are highlighted in Red, it means you cannot. Click on the package you want to upgrade or downgrade the account to and click the upgrade/downgrade button below.
On the overview page, it will show the changes made to the account from either the upgrade or downgrade in specs.
That's all! Good Luck!