1. Please take a look at this Stack question. It points to a tutorial blog on how to do this.
In SPD open default.master, then add the script:
$(document).ready(function()
{
var form = $('form');
if ( form.attr('action').indexOf('YOUR 36 character List ID - Without Braces') > -1 )
{
var checkbox = $('#ctl00_PlaceHolderMain_UploadDocumentSection_ctl03_OverwriteSingle');
checkbox.removeAttr('checked');
checkbox.parent().hide();
var multiple = $('#ctl00_PlaceHolderMain_UploadDocumentSection_ctl03_UploadMultipleLink');
multiple.hide();
}
}
);
2. There is another question on Stack here, where Falak Mahmood suggests that manually changing the Upload.aspx in 12/14 hive is not recommended. Instead, he suggests:
...to write a custom Feature which has AdditionalPageHead delegate control to un-check the "Overwrite existing file" (using jQuery) option while uploading the documents.
