First off, what version of Excel? Excel2007+ use an XML file format, while Excel2003- used a proprietary binary format... so the tools to read and write these work in totally different ways.
If you're after the more recent xlsx files, then take a look at Eric' Gazoni's openpyxl project. The code can be found on bitbucket. The driving force behind this is the ability to read and write xlsx files from Python within a single library.
Even then, it reads the entire workbook, but allows you to modify cells before writing it back.
Simply put: the structure of an Excel file doesn't lend itself to easy editing.... it's not simply a case of changing a few characters.
