title: mutate tibble for particular rows specified by a numbers
body: <p>Trying to find <strong><code>dplyr</code> canonical</strong> way to <code>mutate</code> a <code>tibble</code> by row number.<br />
  I went through many Stack Overflow questions and GH issues, functions like <code>row_number</code>, <code>if_else</code>, <code>case_when</code>, but it is still not clear what is the proper way to achieve the following.</p>
  <p>Minimal example: having a data.frame, I would like to update one of its columns to a particular value (here <code>NA</code>) for particular rows by providing row indices (here rows 2 and 4).<br />
  Column to update doesn't have to be parametrized, but only row numbers.<br />
  Below base R to achieve the following.</p>
  <pre class="lang-r prettyprint-override"><code>DF = data.frame(x=5:1)
  idx = c(2L, 4L)
  DF[idx, &quot;x&quot;] = NA_integer_
  DF
  #   x
  #1  5
  #2 NA
  #3  3
  #4 NA
  #5  1
  </code></pre>