Apex does not have the ability to read or write xls/xlsx files natively. While technically possible, it'd be a massive undertaking and end up being fairly limited in what it could do.
An xlsx file, for example, is an xml document adhering to the "openDocument" format, which is compressed as a zip file (which in turn is a combination of the LZ77 and Huffman compression techniques together with some interleaved data to tell you things like how many files there are, where one file ends and another starts, etc...).
On top of that, you need to worry about governor limits (particularly heap space and cpu time). There appear to be a few Apex zip utilities floating around on github, but I seem to recall that they can barely handle files around 1 MB in size, and that's only decompressing or compressing (not both).
The realistic solution here is to find, research, and purchase a third-party solution (likely doing the bulk of the work off of the Salesforce platform) that allows you to update values through an API.