Go to "settings" (cog shaped icon)
Then type "screensaver" into search box near top and a couple of choices will appear, click on the "switch screensaver on or off" option and a new dialogue box will appear. You may find this dialogue box will be hidden by the settings screen so minimise the settings screen and you will see this little box that will give you all the options you need to switch on and change the settings for the screensaver.
Rupert, March 2020