I want to change a variable in html by js but am not sure how to do it

Here is the line i am trying to change.

<a class="weatherwidget-io" href="https://forecast7.com/en/n45d88170d50/dunedin/" data-label_1="DUNEDIN" data-label_2="WEATHER" data-theme="original" >DUNEDIN WEATHER</a>

I am trying to change the “original” data-theme="original" with “dark” when a function is triggered.

Here is my function

function darkmode() {
   
    document.body.classList.toggle("dark-theme")
    document.body.classList.toggle("logo_paw_dark")
    document.getElementsByName("data-theme").innerHTML = "dark";
    
}

I tried getting the element by the name but did not work and neither did with using id