Assuming the Excel file is in .xlsx, you could rename that to .zip, unzip the file, find the XML for the sheet you want, edit that with a text editor, and then reverse the steps to build the .xlsx back again.
The file you want to edit is in folder xl\worksheets_rels in file sheet1.xml.rels (replace "sheet1" with the name of the sheet).
Assuming the Excel file is in .xlsx, you could rename that to .zip, unzip the file, find the XML for the sheet you want, edit that with a text editor, and then reverse the steps to build the .xlsx back again.
The file you want to edit is in folder xl\worksheets_rels in file sheet1.xml.rels (replace "sheet1" with the name of the sheet).