JSFiddle - React, Tailwind, and code Playground

HTML

<div class="dropdown-plans">
    <select id="basic_plan" name="bill_cycle">
        <option value="tri">3 Years - Rs. 100/month</option>
        <option value="bi">2 Years - Rs. 200/month</option>
        <option value="ann">1 Year - Rs. 100/month</option>
    </select>
</div>
<div class="button-plans">
<a id="abc" href="something"> Order now </a>
</div>

JavaScript

var sel = document.getElementById('basic_plan');
sel.onchange = function () {
    document.getElementById("abc").href = this.value + ".html";
}