This is a guide on how to update your iPhone to iOS 5.1 Beta 3. Note: This works without having a developer account and does not require a UDID either. This solves many users' problems with battery issues. Very simple guide for a very simple process. NOTE: This update blocks the Corona exploit used to jailbreak certain devices so if you are planning on jailbreaking, DO NOT UPDATE. Also, this update brings back the "Turn off 3G" switch that many users wanted.
EDIT: The actual iOS 5.1 has been released. To find out more information about new features, go here.
Step 1) Head over to imzdl.com to download the latest beta for your device.
Step 2) Plug in your device and go to it in iTunes.
Step 3) Hold down Option on the Mac or Shift on Windows and click on Check for Updates.
Step 4) Find the .ipsw that you just downloaded above and select that.
Step 5) The iDevice will now update to the newest beta!
Post in the comments if you have any questions, I will be sure to help you out!