My Mac is OSX El Capital. Does updating Safari affect this in any way?

If you are on Mac OS X El Capitan 10.11.6, that is the last version of El Capitan and Safari will be at the latest/ last version for that OS. Mac OS X El Capitan does not receive updates from Apple any longer.

To get a more modern version of Safari with the latest security then you would need to upgrade your OS

to macOS High Sierra, macOS Mojave or the latest OS macOS Catalina.

You may consider another secure browser and email client for use in El Capitan 10.11.6. Those

supplied by Apple & no longer see 'secure updates' or upgrades in their respective functions.

The following from Mozilla work for free; and may buy you time. Firefox & Thunderbird.

• Firefox:

https://www.mozilla.org/en-US/firefox/new/

• Thunderbird:

https://www.thunderbird.net/en-US/

(Other secure web browsers and email clients may still be available for El Capitan 10.11.6.)