Critical Damage Multiplier Macro

I have an npc I'm working on that has an increased damage multiplier on its damage. I can't seem to figure out how to change this in the attributes tab of the npc character sheet. I might need an api but I've scoured roll20 forus and its reddit and cant seem to find an answer. Can someone help? 

Hi Chris H.,  Which character sheet are you using? 

2014 5e by r20

The NPC version of the D&D 5e 2014 by Roll20 sheet cannot have a different critical hit damage on the sheet itself.  You have two options:  1) Put in a note in the description to roll or state what the damage is on a critical hit.  2) Create a macro for the attack and in the macro put the modified critical hit damage, then use the macro instead of the sheet's button.