When you open "Manage Crest" you will see that you have different positions on top of that tab . One is "convert crests". To do this you need 4 crests of same talent, from same lvl, but on 4 different slots. Then you can convert all of them into 1 Insignia. Just know, that if hero have Insignia, and you will give him crest, Insignia will be removed. Either crests or Insignia, one at time.