title: Pandas - Replace values based on index
body: <p>If I create a dataframe like so:</p>
  <pre><code>import pandas as pd, numpy as np
  
  df = pd.DataFrame(np.random.randint(0,100,size=(100, 2)), columns=list('AB'))
  </code></pre>
  <p>How would I change the entry in column A to be the number 16 from row 0 -15, for example? In other words, how do I replace cells based purely on index?</p>