title: Change values from one column higher than X but not change the others
body: <p>How can I change all values from a column higher than 30 in 30, but if there are lower keep the value that it has?</p>
  
  <pre><code>Day Temperature
  1            25
  2            32
  3            28
  4            35
  </code></pre>
  
  <p>I want to have</p>
  
  <pre><code>Day Temperature
  1            25
  2            30
  3            28
  4            30
  </code></pre>
  
  <p>I have tried with ifelse but it also changes the other values. </p>
  
  <p>Thanks in advance</p>