Hi and welcome to the Forum!
What is happening when you try to download it again, do you get a message? I don't know whether there is a problem with the update or not, but sometimes during an update things just go wrong.
Have you tried the 3 steps when something does go wrong, which is to close your apps completely and then relaunch them and a Restart and/or Reset.
From the home screen do the four finger swipe up or double press the home button to reveal your multi task bar. These are all the apps that you have opened at one time. Long press the app until it jiggles and a red minus sign will appear above it. Press the minus sign, this will close the app, not delete it, now press the home button to return to your screen. It's also a good idea to close all these apps, as it helps the iPad run smoothly, now see if it will download.
Restart - Press and hold the power off button, the one at the top right. You will see a slide appear to power off, slide this and the screen will go dark and a Whirly will appear. When the Whirly disappears your iPad is now off. Press the power button again until the Apple logo appears, this will take a few seconds and your iPad will restart. You should restart your iPad occasionally also as it will keep it running well. Now see if it is operating.
Reset - Hold the power off button and the home button simultaneously, keep holding them in and you will see the power off slide appear and disappear, wait until you see the Apple logo appear before releasing the buttons. It will take a little longer for your iPad to start. Let us know how you go.
