These are general instructions. You can get instructions specific to your Domain Host.
Open DNS Management for the domain you want to edit (aka Domain Management, Name Server Management etc).
You might have to enable advanced settings to access this page.
Locate the Host Name / Alias field
Set the Value / Destination to one of the following LaunchRock IP addresses.
*To increase reliability you should create four A Records for your domain and point each one to a different LaunchRock IP address
Once you've modified your A records, it may take up to 72 hours for changes to update live.