from openpyxl import load_workbook

path = 'valami.xlsx'
wb = load_workbook(path)
sheet = wb.worksheets[0]
sheet.cell(row = 1, column = 1).value = 35
sheet.cell(row = 2, column = 1).value = 42
sheet.cell(row = 3, column = 1).value = 16
sheet.cell(row = 4, column = 1).value = '=sum(a1:a3)'

wb.save(path)