how do you download safari 11?

Safari 11 is not a standalone application that you can download.

Safari 11 is part of the macOS High Sierra installation, with minor updates to Safari becoming available with regular updates to that operating system. If you have Sierra 10.12.6, or El Capitan 10.11.6 installed, then you are also eligible to receive Security updates for those two operating systems — that will include Safari 11, and its minor revisions. These will appear in the Mac App Store Updates tab.

I have applied all Security updates after 10.11.6 was released, and now have Safari 11.1.1.

Once macOS Mojave is released, it is likely that El Capitan (10.11.6) Security updates will disappear from the second paragraph.

Open the Mac App Store, click on the Updates tab, and install any Safari or security updates listed.

(161491)