Is it possible?
I'm really sorry but we don't follow you well.
Please try to explain in other words or it will be better using some screenshot.
Please try the following CSS code:
.wpf-bell .fa-bell:before{
content: "\f002" !important;
}
Please note the red marked value the icon content value and it can be changed.
The CSS code should be added in the Dashboard > Forums > Settings > Styles admin page "Custom CSS Code" textarea.
Also please check the following link it may help you: https://astronautweb.co/snippet/font-awesome/
Thanks a lot! And i have another question, can i put my own icon/image?
I was reading your URL but my english is not too good
Please follow this support topic. Here wpForo members share their experience on how to replace Font-awesome Icons with Images: https://wpforo.com/community/how-to-and-troubleshooting-2/replace-font-awesome-icons-with-images/