My old MacBook is still running High Sierra very happily, but now I've got this big download in Applications and a nag notification to upgrade that won't go away unless I click on "Details". I don't need to - or want to - upgrade, so can anyone tell me how to get rid of the notification and stop the thing from downloading itself again? I've found various hints on the net, but none of them seem to work, and it seems unfriendly (to say the least) not to have a "no thanks" option to stop it bugging me.