I assume your colleague is sending you a packaged workbook (.twbx) file? If so, it's possible that you might have a copy of the original Excel or text file and not even know it! .twbx files are really just zip files that contain the workbook (.twb) and the data (often as a .tde extract, but if your collegue used a live connection to Excel or csv it might contain a copy of the file). So, just rename the .twbx to .zip and then see what it contains.
Now, it's quite possible that your colleague extracted the data and it will just contain the .tde file. If so, then there isn't really a good option for editing the data in Tableau (it is a data visualization tool afterall, not a data editing tool). If you just want to edit values, you could possibly use calculated fields. But likely, your work-around is probably the best option (or see if you can convince your colleague to use live connections).
Hope that helps! Welcome to the forums!
What do you mean by "...to change up the visualizations."? Because I'm wondering if you can use calculated fields as a way to create the necessary fields you need.
If the source file is on a network drive then have the author use URL naming of data sources: \\corp\shared\personal\Ann\my_excel_data.xls instead of M:\personal\Ann\my_excel_data.xls .