iOS 6.1.3 delete update to iOS 9.3
I'm not totally sure what you're trying to do, but it sounds to me like you're trying to upgrade an iPad over-the-air. This should work, but transcending multiple operating system versions on your way up can cause problems. And could you be out of storage space?

Since this method isn't working, you should try the other method (scroll down after opening that link), which is updating the iPad via iTunes. 


Plug the iPad into a Mac with iTunes installed.
Make the devices trust each other (accept trust if prompted)
Ensure the iPad is backed up to either iCloud or iTunes.
When the prompt is displayed on the Mac ("Would you like to update the software on this iPad") choose Download and Install to start the update.


Let the iPad update, and you'll be good to go.