Hi @bill
You can do this by hooking into acf/save_post action hook. Then just retrieve your newly saved meta field and modify the post title using wp_update_post.
http://www.advancedcustomfields.com/resources/acfsave_post/
You might also want to read up on wp_update_post here and specifically the Avoid infinite loop section since that will be applicable to your situation.
https://codex.wordpress.org/Function_Reference/wp_update_post
