Can I update my iPad 5.1.1 to 9?

Depends what exact model of iPad you have.

If you have a first generation iPad (no cameras) then it cannot be updated to anything beyond iOS 5.1.1

If your iPad does have a camera then it can be updated to at least iOS 9.3.5

Follow the steps here to see if you can update:

Update your iPhone, iPad, or iPod touch - Apple Support

It depends which iPad model you have.

iPad 1st generation max iOS 5.1.1

iPad 2nd/3rd generation max iOS 9.3.5

iPad 4th generation max iOS 10.3.3

iPad Air/ Air 2 the latest iOS 11.2.6

and so on....

Sir, maybe you can help me. I have an older iPad Model A 1490 with OS 11.2.6.

I need to re-install the OS as we are having a software issue whereby the Wi-Fi does NOT stay connected after I close the cover or put it to sleep. The Wi-Fi turns OFF and when I awaken the device I need to go to settings: Wi-Fi not connected > then I reconnect. Please help!

Yes, that is correct. The problem is when we "wake" the iPad up again by opening the cover, the Wi-Fi does NOT wake up and status is "Not Connected". It used to reconnect automatically and now it does not. I believe there is a glich in the download done a few days ago when we upgraded to 11.2.6. So how do we go about and RE-INSTALL 11.2.6 to possibly solve this issue.

I found the problem. In OS 11.2.6 there is a parameter which permits the user to Turn Off or ON the Automatic LogIn of the Wi-Fi after the iPad has gone to sleep. IT WAS SET TO OFF. All I had to do with click to ON. When the device "wakes up" it automatically logs back on to the Wi-Fi network. Thanks for your insights though.

You have an iPad mini 2.

All iPhone and iPad switches Wi-Fi off in sleep mode to save battery.

Connect your iPad to wall power to avoid that. (It will keep Wi-Fi alive)

It has nothing to do with the iOS 11.2.6 update

Reboot your router, check if you have new router firmware update from the manufacturer.

FWIW, my iPad Air 2 and iPad Pro run fine on Wifi with iOS 11.2.6

If it was a router problem, all my other devices would have been giving problems also. Every other divise was working correctly. It had to be an issue of the iPad.