This update should appear in iTunes when you plug in your iPad; whether it is a WiFi or WiFi+3G version.
If you do not see the update in iTunes you can click on the Update button in the Summary pane with your iPad plugged in. Still having problems getting this massive 456.9MB payload from Apple? You can try getting it directly from Apple Support page.
If you opt for the manual method, with the iPad connected to your computer, go to iTunes’s Summary pane for the iPad hold down the Option key (OS X) or Shift key (Windows), then click on the Update button while the iPad is connected to the computer.
Unfortunately, this is also a minor iOS update for the iPad, we will have to wait until the Fall 2010 to see the iOS 4.x update for the iPad. I hope Apple will align the iOS versions on the iDevices this Fall. The last thing Apple wants to do is eliminate their wall-garden advantage over Android OS’s disparate hardware and manufacture.