The backslash is used as the "escape character" in quoted strings. It tells FileMaker: "use the very next character as just a character, not the character to end a quoted string.
so "\" is evaluated as the start of a quoted string, (first quote) that contains a double quote character (backslash quote) and then you get an error because no quote is found to end the string. To get a backslash, you can escape it with another backslash
"\\" will evaluate as a single backslash character .
But I think that your calcuation to use in Replace Field Contents--which will modify every record in your found set BTW--is this expression:
Substitute ( YourTable::YourField ; "\\" ; "/' )
You can also build this calculation directly into your Web Viewer instead of doing a replace on your table.