If you do mean iOS 5 - you do it the same way that you updated the iPad to iOS 4.
Plug the iPad into into the computer and launch iTunes. Select the iPad on the left side under devices and then click on the Check for Update button in the Summary pane on the right side of iTunes.
When iTunes launches with the iPad plugged in, it should alert you that the update is available and guide you from there.
There is no current iOS version of 4.0.1. To upgrade to the current version of iOS which is 5.0.1 start your computer run iTunes. Check for updates for iTunes and make sure you have the latest version. If you do not then download and install the latest version of iTunes. Once that is running connect your iPad to you computer, make a backup of your iPad and then follow the instructions for updating to iOS 5.0.1.