Clicking "Upgrade Plugins" doesn't work

Hi guys. One of my sites is on 2.8.1 and I have been advised to upgrade. Weirdly, clicking “Upgrade Plugins” doesn’t work. It just returns me to the page again after doing a quick http trip.

How would you like me to upgrade? FTP the unzipped contents? I dare not do anything rash in case there’s a script that runs that I am unaware of.

Kind regards,