You can either delete enough stuff that you have the necessary space to update via iCloud, or you can update using iTunes and a computer; which requires far less free space on the iPad.
If you have a lot of pictures and videos, save them off the iPad then delete. If you have apps that you seldom use, and contain no useful data, delete them. You can always download them again.
People have had a lot better luck using iTunes and a computer with this update; though I used iCloud without issues, and newer iPads are less likely to have issues with the iCloud update.
Strange that it's gone up to over 6GB. It was only a bit over 3GB with the first few update versions. I wonder what changed.
Don't forget to do a backup first.
Also, restarting the iPad just before updating is a good idea. Then restart or reset it afterwards for luck. It seems to help