https://newsammo.com/wp-includes/images/rss.png

usually, I create a replacement and name the replacement like for like (in this case rss.png), rename the current image file to something like old-rss.png in case need to revert.

I also found this method using CSS, but their code didn’t work quite right for my site: https://en.forums.wordpress.com/topic/change-color-of-icon-in-rss-widget/

Here is the icon I’m trying to use:https://newsammo.com/wp-content/uploads/2019/01/iconfinder_United-States-of-AmericaUSA_16036.png

try below:

Dashboard >> Appearance >> Customise >> Additional CSS

.rss-widget-icon {display: none;}

.swidgets-wrap .sidebar-title-border h3{
	background-image: url("https://newsammo.com/wp-content/uploads/2019/01/iconfinder_United-States-of-AmericaUSA_16036.png");
	background-repeat:no-repeat;
	 background-position: 1px -3px;
	padding-left: 30px;
	color: #ff0000 ;
	}

.swidgets-wrap h3 a{color: #ff0000 ;
	}

First part simply asks not to display the icon (https://newsammo.com/wp-includes/images/rss.png)

Second part adds your flag in your title area as a background image, please change the hex code for the color to what you want for your text (#ff0000 is red just as an example), do same for third part too

Any problems, just remove from the Additional CSS and it should revert to previous state.

probably not the most efficient coding but seems to give the outcome you’re looking for