As long as $team_code is a post ID (Integer) that represents the post you want to set the field and $post_id is the post ID of the post where you want to set this value and you use the field key for the field and this field is not some type of a sub field, like a field in a group field, or a clone field, or a repeater or flex field then
update_field($field_key, $team_code, $post_id);
should be updating the field.
If the field is being set in the DB and your not seeing it on the front end, do you see this change in the admin?
Do you have any caching plugins installed that would prevent this change being reflected on the front end?
