Is there a way to set NPC sheets to crit on 19-20?

Just wondered if there was a way to set NPC sheet attacks to crit on numbers less than a natural 20? I know it can be done on Character sheets, but NPC sheets have no such field.

If it's the D&D 5e by Roll20 (2014) sheet, then you will need to convert the NPC sheet into a PC sheet in order to change an attack's crit range. Or use what keithcurtis described below. When asking these types of questions, it's a good idea to mention which game system and which of that game system's character sheets you are using. Although D&D 5E is probably the most popular game system used on this site, there are dozens of others, with many having multiple sheets to choose from.

Hi Shard! Assuming you are using the D&D 5e (2014) Sheet, then no, this has not possible, though it has been asked several times int he past. The only real option would be to rewrite the attack as an Ability on the Attributes and Abilities tab.

To convert an NPC character sheet to a PC character sheet, go to the settings cog at the upper right of the NPC sheet, then scroll down to where it says "Character Sheet Type" click on "NPC", then select "PC" from the drop-down list.

I would do what Keithcurtis suggested and make it an Ability macro.  To do that, make an attack.  Then click the chat box in the Chat tab (where you type).  Hit the up arrow on your keyboard.  Copy the text, it will look similar to this:  @{Abominable Yeti|wtype}&{template:npcfullatk} {{attack=1}} {{damage=1}} {{dmg1flag=1}} {{dmg2flag=1}}  @{Abominable Yeti|npc_name_flag} {{rname=Claw}} {{r1=[[ @{Abominable Yeti|d20} +(11+0)]]}} @{Abominable Yeti|rtype} +(11+0)]]}} {{dmg1=[[2d6 + 7+0]]}} {{dmg1type=slashing}} {{dmg2=[[2d6+0]]}} {{dmg2type=cold}} {{crit1=[[2d6+0]]}} {{crit2=[[2d6+0]]}} {{description=}} @{Abominable Yeti|charname_output} Change the first bolded field to: 1d20cs>19 Add to the second bolded field cs>19 This way they read: @{Abominable Yeti|wtype}&{template:npcfullatk} {{attack=1}} {{damage=1}} {{dmg1flag=1}} {{dmg2flag=1}}  @{Abominable Yeti|npc_name_flag} {{rname=Claw}} {{r1=[[ 1d20cs>19 +(11+0)]]}}  @{Abominable Yeti|rtype}cs>19 +(11+0)]]}} {{dmg1=[[2d6 + 7+0]]}} {{dmg1type=slashing}} {{dmg2=[[2d6+0]]}} {{dmg2type=cold}} {{crit1=[[2d6+0]]}} {{crit2=[[2d6+0]]}} {{description=}} @{Abominable Yeti|charname_output} Then put that into an Ability macro and name it, set it as a token action.