9.3.5 Update Not able to install

I have an Iphone 5c and it won't let me back up or install the new update 9.3.5. Therefore, it has made my phone respond slower and now has removed all of my text messages. I am unable to view any previous text messages. I have enough memory on my phone for the update. I've turned the phone off to re-boot. And I plugged into a computer to add the update and nothing. What do I do?