Mostly theme's use
wp_enqueue_style() function inside their functions.php file to add style sheet in the header. Here is how to find out if your theme does the same.
wp-content/themes/YOUR_THEME_NAME/functions.php file, and find out the line which is adding the style sheet, Like:
wp_enqueue_style('main_style', get_stylesheet_directory_uri() . '/style.css');
wp_enqueue_style( 'twentysixteen-style', get_stylesheet_uri() );
You can search for the ID (except for
-css part)... if the ID is:
main_style-css search for just
main-style in your functions.php file, and you should probably find the line of code you were looking for.
Now that you found the code and you know that your theme adds this stylesheet by using
wp_enqueue_style() in functions.php file. You need to update this code for version.
$style_ver = filemtime( get_stylesheet_directory() . '/style.css' );
wp_enqueue_style( 'main_style', get_stylesheet_directory_uri() . '/style.css', '', $style_ver );
As you can see, this code gets the last modified time of style.css file using
filemtime() PHP function and it also converts the time to timestamp using
time() PHP function just to make things clean.
If you don't want the version to dynamically change every time you can simply do this:
wp_enqueue_style( 'main_style', get_stylesheet_directory_uri() . '/style.css', '', '1.5' );
Thats pretty much it. Peace!