Solved with a typo issue.
Finally got it by using:
$value = “My new value”
$field = get_field_object(‘name_of_field’,$postId);
$field[‘custom_field_name’] = $value;
acf_update_field($field);
I had to resolve this by “updating” the fields in the function where i was creating the post. This then “Activated” the fields and allowed them to be updated.
Ok I got this to work as needed as a multi select. I’m currently using product categories taxonomy, is it possible to get the multi select to pick up the woo products instead of product categories?
Ah perfect that sorted it thanks!