Well popular courses are fetched on basis of number of students on site .
If you want custom order in it then please goto wp-admin -> wplms -> course manager -> "
Maintain accurate Student Count " : no .
then set the number of students manually from edit course page,find custom fields and then find "vibe_students" ,change value and then click update below it .
refer :
http://prntscr.com/hoxs5p
.