Modify Post Title and Metabox Field Values before save

I would like to modify 'post_title' and Metabox fields in either the 'rwmb_frontend_validate' filter or the 'rwmb_frontend_before_process' action but when I try to change $_POST['FIELD_NAME'] = 'NEW VALUE' there is no change.

I'd also like to programatically add a new term to a custom taxonomy field in a filter or hook before processing.

Thanks