I did the update. Took less then 10 minutes to do. A few things to remember if you are doing an OTA (over the air) update is to make sure it is plugged in to a power source first and that you are on WiFi. Not sure if either one of those is a requirement, but they are definitely highly recommended.
You can always plug it into your computer and update it that way. That is usually faster, tho I was impressed that this took less than 10 minutes from start to finish. The update is 54mb, so your time may vary depending on your internet connection.
To perform the OTA update Click Settings> General > Software Update > Download and Install. You will have to agree to their Terms of Service and then sit back an relax. It will download the update, install it and restart the device.
If you were already on version 5.1 you probably won’t notice any improvements. According to the information, there are no feature updates, but instead performance updates and bug fixes.