How do I change the IP through WHM for a domain name? My client needs an assigned IP to their domain.

Try this:
Log-in as root into the WHM
https://yourdomain.com:2087

Then:

Main >> Account Functions >> Change Site's IP Address

I'm a reseller my WHM is located at https://yourdomain.com:2086 and I do not see the "Change Site's IP Address" option in the area you sepecified.

Main >> Account Functions >> Change Site's IP Address

You will need to ask your provider to change this for you then, also you should be using 2087 not 2086 for a secure port. (Was not sure if it was a typo or not seeing the 'https' but figured I would mention it)

How do I change the IP through WHM for a domain name? My client needs an assigned IP to their domain.

You may need root access for this.
http://www.cpanel.net/docs/whm/Change_Site_s_IP_Address.htm

Since I'm a reseller, I do not have access. I've got it straitened out now. Thanks everyone.