It depends on your theme. Some you just have to know this block's ID (15, 12, 102 or other) and get in a code like this:
html[dir="ltr"] .cForumList .cForumRow[data-categoryid="3"] .cForumTitle::before {content: "\f005" !important;}
We put this on Fluent/Design theme, in Edit and Forums. There's a block named [Icons Category] CSS code.
So, if you use nexxe theme, this is how you change:
In nexxe theme you go to Edit, Forums and then cb__lang_forums_icons.
So, just put this code, otherwise, it won't work:
ol#cb-nexxe__category > li.cForumRow[data-categoryid="13"] .ipsType_sectionTitle:before { content: "\f651"; }
But that's how I've said, it only works if you know your theme's code, but… This usually can be found in Edit, in your AdminCP → Themes.