title: Fill the Null values of the first row of dataframe with 100
body: <p>I have a dataframe which looks like this:</p>
  
  <pre><code>51183 53423 51989 52483 51342
   100   NaN   NaN   83.33  NaN
   NaN   NaN   50     25    12.5
  </code></pre>
  
  <p>Here , '51183' , '53423'....are column names. I want to fill the null value present in the first row with 100.</p>
  
  <p>I tried doing this:</p>
  
  <pre><code>df[:1].fillna(100)
  </code></pre>
  
  <p>It just changes the null values in the first row to 100 but it doesn't update it in the dataframe.</p>
  
  <p>I want the result to look like this:</p>
  
  <pre><code>51183 53423 51989 52483 51342
   100   100   100   83.33  100
   NaN   NaN   50     25    12.5
  </code></pre>
  
  <p>If you could help me achieve that , I'll greatly appreciate it.</p>