There isn’t an easy way to do this. You will need to
1) Get all the posts of the post type
2) Check to see if the field already has a value
3) update if it doesn’t
Depending on the number of posts involved, this process could very well time out before completion.
The better alternative is to set a default value if the field does not contain a value.
$value = get_field('field_name');
if (!$value) {
$value = 'set some default value';
}
The checking of this may need to be more complex, but that’s the idea.
Alternately you could add an acf/load_value filter to the field and return a default value from this and this would not require altering templates where it’s used https://www.advancedcustomfields.com/resources/acf-load_value/
