How to edit div class with capybara - html

Question

I have a (I hope) simple question. Is there a way to change something like
<div id='THE_ID' class='filter_group'> subtags </div>
to
<div id='THE_ID' class='filter_group open'> subtags </div> in capybara?
This website I'm looking at has a non-descript <a> tag as a button that edits the div class as seen above.
I'd rather click the <a> tag but it being non-descript has no information. No name, no id, no href, nothing.
I'm new to rails and capybara so please forgive me if I'm missing something simple
The code block I am looking at is
<div id="ctl00_ContentPlaceHolder1_as1_divDateRange" class="filter_group">
<label class="header">Date Range
<span class="label two-line"><span id="ctl00_ContentPlaceHolder1_as1_lblDateFrom">4/14/2018</span><br><span id="ctl00_ContentPlaceHolder1_as1_lblDateTo">6/14/2018</span></span>
<a></a>
</label>
<div class="list">
RADIOBUTTONS and FIELDS
</div>
</div>

Answer 1

What you're asking is possible using JS, however then you're not really testing the site (assuming you're testing and not just scraping). To do it via JS with Capybara 3.2+ you could do
page.find("#ctl00_ContentPlaceHolder1_as1_divDateRange").execute_script("this.classList.add('open');")
Prior to 3.2 you would need to do something like
el = page.find("#ctl00_ContentPlaceHolder1_as1_divDateRange")
page.execute_script("arguments[0].classList.add('open');", el)
That being said, there are plenty of ways to click the <a> even if it doesn't have any usable attributes (assuming it has size on the page) by scoping to it
page.find("#ctl00_ContentPlaceHolder1_as1_divDateRange > .header > a").click

Answer 2

Here is some code (Sorry it's pure javascript I didn't see the part of your question where it says it's ment to be with cappybra):
function yourFunction() {
var element = document.getElementById("ctl00_ContentPlaceHolder1_as1_divDateRange");
element.classList.add("open");
}
.open {
color:red;
}
<button onclick="yourFunction()">Add class</button>
<div id="ctl00_ContentPlaceHolder1_as1_divDateRange" class="filter_group">
<label class="header">Date Range
<span class="label two-line"><span id="ctl00_ContentPlaceHolder1_as1_lblDateFrom">4/14/2018</span><br><span id="ctl00_ContentPlaceHolder1_as1_lblDateTo">6/14/2018</span></span>
<a></a>
</label>
<div class="list">
RADIOBUTTONS and FIELDS
</div>
</div>
The CSS part is to demonstrate that the class has beeen added.
Hope I helped you and others :)