Please refer tip: https://wplms.io/support/knowledge-base/add-custom-information-to-custom-fields-in-course-detail-section-with-different-icons-on-each-field/
This tip has been updated now.
If you want to change the plus icon, then change the class 'icon-plus-1' to your desired icon class. For icon class refer: yoursite.com/wp-content/themes/wplms/css/icons-reference.html