■openpyxlのインストール
PS C:\sample> python3 -m pip install openpyxl
■コード
import openpyxl as px
filepath = r'c:\sample/test.xlsx'
savepath = r'c:\sample/test2.xlsx'
wb = px.load_workbook(filepath)
ws = wb.get_sheet_by_name('Sheet1')
ws.cell(row = 1, column = 1, value = "あああああ")
wb.save(savepath)