Can I update my iPad to 9.3.1?
It's not the same as your but I have tried mine just few days back... it's iOS 8 and iPad3.  It was okay but had to be connected to iTunes (USB).

Over the air had problems.
Using non-original cables will also cause issues (my previous experiences).

Just make sure you make a backup as there is a big chance it will fail and you have no choice but to restore from backup which will first update your iPad to the latest OS then re-download the software and data.