1. Go to WP ADmin -> Plugins -> Editor -> WPLMS Customizer -> customizer_class.php
2. Add the below code in _construct function 

PHP Code:

 

add_filter('wplms_unit_types',array($this,'custom_unit_types'));

 

3. Add below function in the class : 

PHP Code:

 

function custom_unit_types($unit_types){
  $unit_types[]=array('label'=>'Custom','value' => 'icon');
return $unit_types;
}   

 In above Label is the name you see in the select box for unit type and replace the icon from this link (scroll to the bottom of the page and copy the icon name) :

http://vibethemes.com/envato/wplms/wp-content/themes/wplms/css/icons-reference.html