Save Custom Fields on Custom Post Type Sub Menu Page

I have a custom post type named playlist in which an audio playlist is built from attachments. Each audio attachment has custom fields. So in my custom post type the audio is uploaded and titles are displayed in a list as they are added. Each title has an edit button that links to a hidden sub menu page where I would like to be able to update the custom fields for the audio. I have the fields in the sub menu page already but im not sure how to save them. Does anyone have any ideas here? I would think that it would be somewhat like saving any other custom field but i’m not seeing how that is possible. Thanks for any help.