title: Replace first row in specific columns with 0
body: <p>I want to replace the first row of two columns with <code>0</code>. I can achieve this by returning a new <code>df</code>. But it doesn't work when changing <code>inplace = True</code>.</p>
  
  <pre><code>df = pd.DataFrame({   
      'Code1' : ['A','A','B','B','C','C'],    
      'Code2' : [np.nan,np.nan,5,np.nan,np.nan,10],                      
      'X' : [np.nan,np.nan,1,np.nan,np.nan,3],    
      'Y' : [np.nan,np.nan,2,np.nan,np.nan,4],                                   
      })
  
  
  df.loc[:0, 'X':'Y'].replace(np.nan, 0, inplace = True)
  </code></pre>
  
  <p>Intended Out:</p>
  
  <pre><code>  Code1  Code2    X    Y
  0     A    NaN  0.0  0.0
  1     A    NaN  NaN  NaN
  2     B    5.0  1.0  2.0
  3     B    NaN  NaN  NaN
  4     C    NaN  NaN  NaN
  5     C   10.0  3.0  4.0
  </code></pre>