How to update IOS 8.1.3 on an old mini-IPad

I have just found my old mini-IPad which had been misplaced for three years. It has IOS 8.1.3. When I tried to update it by going to General/Software Update, I got to the page of Download and Install IOS 9.3.5. But nothing happened after I clicked Download and Install. I tried multiple times to no avail. Would appreciate any advice on how to update to more current IOS. Thank you.