How to replace an image without HTML changes?

Question

JavaScript

Hello! I'm a complete JS noob and need help. Googled a lot, but apparently my knowledge is not enough. The point is this. There is an image added to html and it has no id and only class. How can I use JS or jQuery to replace the image
Here is the html snippet

div style="float: left; margin-right: 10px">
<img width="50" class="user-profile-image user-default-profile-image " src="img/default_profile_50.png"></div>

Tried to assign an id with

function myFoo1(){document.getElementsByClassName("user-default-profile-image")[0].id = "defimage"; }

And replace with

$('#defimage').attr('src', '/fileservice/file/download/a/233337/sc/494/h/c803022e0f37921830b75a0d243e73e0.png')

But it doesn't come out