how to update from Safari 15 on iMac Late 2013
using MacOS Catalina 10.15.7
Question marked as
You cannot. Safari 15 is the most up to date version Catalina can run. And a 2013 iMac cannot update beyond macOS Catalina to get a newer Safari version.
Your options are either finding a different browser that still works on Catalina, or getting a newer computer than can run a newer macOS version and as such a newer Safari version.
The current version of Firefox still works on Catalina.
https://www.mozilla.org/en-US/firefox/mac/