How to modify single.php in a child theme?

your child theme
can override any file in the parent theme: simply include a file of
the same name in the child theme directory, and it will override the
equivalent file in the parent theme directory when your site loads.

with the exception:

Unlike style.css, the functions.php of a child theme does not override its
counterpart from the parent. Instead, it is loaded in addition to the parent’s > functions.php. (Specifically, it is loaded right before the parent’s file.)

Source

in your case, just copy single.php to your child theme and do your edits.