After purchasing a dedicated IP you can change your domains to it from within your control panel.
Please follow below steps to do it,
1. Login to your control panel
2. Go to Websites & Domains
3. Locate your domain
4. Click Web Hosting Access
5. Select the dedicated IP which you wish to use from the drop-down list at IP Addresses option.
6. Click OK
Note that ALL domains in your account will be switched to this dedicated IP address.