Hi @nitins
1. Use the get_posts function to return all the posts you want
2. Loop over the posts
3. For each post, update the value
To update the value, either use a WP function such as update_post_meta or the ACF function update_field.
Please note that update_field will only work to update a repeater field, not a sub field within a repeater.
Saving a repeater field: http://support.advancedcustomfields.com/forums/topic/update_field-with-repeater-fields-issue/
Thanks
E
