title: switching R version for installing a package
body: <p>I am using R on a mac.</p>
  <p>I am trying to install a package but it is not available in the version that I have.</p>
  <pre><code>Warning in install.packages :
    package 'ETC...' is not available (for R version 3.6.0)
  </code></pre>
  <p>How can I use an older version in order to install this package.</p>