<div class="nav-about">
<li><a href="#block"></a></li>
<li><a href="#block"></a></li>
<li><a href="#block"></a></li>
<li><a href="#block"></a></li>
</div>
<div class="nav-about">
<li><a href="#block1"></a></li>
<li><a href="#block2"></a></li>
<li><a href="#block3"></a></li>
<li><a href="#block4"></a></li>
</div>
const links = document.querySelectorAll('.nav-about a');
links.forEach((n, i) => n.attributes.href.value += i + 1);
// или
for (const [ i, n ] of links.entries()) {
n.setAttribute('href', n.getAttribute('href').concat(-~i));
}
// или
for (let i = 0; i < links.length;) {
links[i].href = links[i].href.replace(/.*(#.*)/, `$1${++i}`);
}