Firstly, you'll need to identify your name servers. You can do this by either checking your client area and navigating to your product information. Otherwise, you can check your welcome email which will contain your name servers.
Next, you'll need to login to your domain control panel (Where you purchased the domain) as that's where your domain information is stored and can be edited from.
Next, you'll need to find the settings for your name servers.
This is the easiest bit, you'll have some current default name servers. All you have to do is change to custom, and edit them. When you can edit them, you just have to copy and paste the original name servers from your email from your hosting company.
After you've added your new name servers and saved. That's all you have to do. It is advised that it could take up to 48 hours for your domain to propagate. However, it's likely it will only take 2-3 hours. Otherwise, you can use a proxy to access your website in the mean time.