Locating the file should be easy - look for any file upload script.
Updating a client side mdb file might be harder but look for some of the code for ole automation with word. Access should follow similar rules.
I'd be inclined to display the records they can download and have each on a separate form with an upload button for each and get the user to them one at a time, alternatively do it using check boxes displayed the way the webmail clients do with a single hit. It doesn't use an array though.
You might be best searching the javascript forums!