title: python: update dataframe to existing excel sheet without overwriting contents on the same sheet and other sheets
body: <p>Struggling for this for hours so I decided to ask for help from experts here:</p>
  
  <p>I want to modify existing excel sheet without overwriting content.
  I have other sheets in this excel file and I don't want to impact other sheets.</p>
  
  <p>I've created sample code, not sure how to add the second sheet that I want to keep though.</p>
  
  <pre><code>t=pd.date_range('2004-01-31', freq='M', periods=4)
  first=pd.DataFrame({'A':[1,1,1,1],
               'B':[2,2,2,2]}, index=t)
  first.index=first.index.strftime('%Y-%m-%d')
  writer=pd.ExcelWriter('test.xlsx')
  first.to_excel(writer, sheet_name='Here')
  first.to_excel(writer, sheet_name='Keep')
  
  #how to update the sheet'Here', cell A5:C6 with following without overwriting the rest?
  #I want to keep the sheet "Keep"
  update=pd.DataFrame({'A':[3,4],
                       'B':[4,5]}, index=pd.date_range('2004-04-30', 
                                                       periods=2,
                                                       freq='M'))
  </code></pre>
  
  <p>I've researched SO. But not sure how to write a dataframe into the sheet.</p>
  
  <p>Example I've tried:</p>
  
  <pre><code>import openpyxl
  xfile = openpyxl.load_workbook('test.xlsx')
  sheet = xfile.get_sheet_by_name('test')
  sheet['B5']='wrote!!'
  xfile.save('test2.xlsx')
  </code></pre>