You choose one of the examples (https://codex.www.ads-software.com/Function_Reference/the_modified_time#Examples) and put the code in a template, such as single.php, and this will show on the site so that people can see the last updated time.
This plugin – Last Modified Timestamp – “Adds the last modified time to the admin interface as well as a [last-modified] shortcode to use on the front-end.” (front end being whatever template you put this in)
From the Plugin’s FAQ:
To customize the output with a shortcode, use the attributes as described above.
…and…
Template Tags
Models the function naming convention used by WordPress for get_the_content / the_content and similar functions.
get_the_last_modified_timestamp()
– returns timestamp.
the_last_modified_timestamp()
– displays/echos the timestamp.