It is a couple of minutes work to do what you require, but could take hours to explain to you step by step what to do. Here is the way I would do it (may not be the best, but any way that works will do).
Using a program like FILEZILLA (it's free, and a tool like this makes life easier for working with WordPress) download the function.php file from the .../wp-content/themes/hueman folder onto your own computer.
Open this function.php file using some text editor program (for example Windows Notepad) or Dreamweaver or similar program.
Basically delete everything out of this functions file
except the bit you want to change,
ie. the alx_setup function.
Edit this alx_setup_function as required.
Save the file, and then use FILEZILLA to upload the file to your
wordpress site, to .../wp-content/themes/hueman-child folder.
The file is still called functions.php
The function in the child theme (folder) will then get used in preference to the same function in the main Hueman theme (which you have not changed -- that is the benefit of using a child theme).
You are not in this case altering the style.css file.
(But if you do later want to make small changes to the CSS, then I would use exactly the same procedure as above (except start with the child style.css file)).
If none of this makes any sense then you probably are in over your head (but could easily find a WP developer to make the change for you for a small sum). If you understand most of it but have a quick question it may get answered here :-)