title: how to use mutate_if to change values
body: <p>How can I use <code>mutate_if()</code> to change values of <code>b</code> to <code>NA</code> in case <code>a &gt; 25</code></p>
  <p>I can do it with <code>ifelse</code> but I feel <code>mutate_if</code> is created for such task.</p>
  <pre><code>library(tidyverse)
  
  tbl &lt;- tibble(a = c(10, 20, 30, 40, 10, 60),
                b = c(12, 23, 34, 45, 56, 67))
  </code></pre>