Changing A critical threat range and critical damage on a macro 5E

Trying to change  %{selected|repeating_npcaction_$0_npc_action} This to increase the dice an extra D4 and crit on 19 and 20

You will need to change the NPC sheet to a PC sheet to change the crit range on an attack, I'm fairly sure. The other option is to manually write a macro for it. Here is an example Giant Spider bite with modified crit range: @{Giant Spider|wtype}&{template:npcatk} {{attack=1}} {{damage=1}} {{dmg1flag=1}} {{dmg2flag=1}}  @{Giant Spider|npc_name_flag} {{rname=[Bite](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_dmg)}} {{rnamec=[Bite](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_crit)}} {{type=[Attack](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_dmg)}} {{typec=[Attack](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_crit)}} {{r1=[[@{Giant Spider|d20} cs>19 +(5+0)]]}} @{Giant Spider|rtype} cs>19 +(5+0)]]}} {{description=The target must make a DC 11 Constitution saving throw, taking the poison damage on a failed save, or half as much damage on a successful one If the poison damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.}} @{Giant Spider|charname_output} Use the following tip from the macros page to pinch the NPC macro you wish to edit, and add the bolded code above in the same place to change the crit range. Tip: You can click on any roll, on your character sheet, then go to the chat window and press the up arrow to reveal the code used. This can be used to combine various rolls into one handy button! What is the 'extra d4', is that damage? If so, just directly edit the attack on the NPC sheet.

The extra d4 is only on a critical is for brutal critical.  I knew that macro trick about hitting up holy crap some macros are large and confusing 

Ahhhh ok, you're going to have to write a macro for the Crit damage as well then, I'm afraid. AFAIK you can't modify Crit damage on those NPC attacks, they just automatically roll double the dice of a standard damage roll. Once you have modified the Crit damage macro to add the extra D4, you would want to save that as an Ability macro on the NPC sheet - using the example from above, we'd save it under the Giant Spider sheet in its Ability section, and call the macro BiteCrit. We then need to change the 2 links in the attack roll to use our Crit damage macro instead of the auto-generated one. Again, this is from the Giant Spider macro I pasted above. We change these 2 horrible looking fields: {{rnamec=[Bite](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_crit)}}     {{typec=[Attack](~-M-EmGcefMq1fS3TnScA|repeating_npcaction_-M-EmH-EYjSePhXqJgxJ_npc_crit)}} into this: {{rnamec=[Bite](~Giant Spider|BiteCrit}}      {{typec=[Attack](~Giant Spider|BiteCrit}} See how much friendlier those links look without the repeating field ID strings in them? The {{rnamec}} field is the big text at the top of the template output which you can click to roll crit damage. The {{typec}} field is the text just to the left of the d20 rolls in the attack roll macro, which is also a link to the crit damage roll. You don't have to make them both links, but that is how the default sheet attacks work. This page is a handy resource for template outputs! If you have any dramas getting it to work, just post back here with the attack you're trying to modify/get working and someone will help out.

This seems like a lot of work for a simple NPC lol

Yeah, it's a lot of work if you've never done it before and the NPC only gets one attack in before it's murdered - I wouldn't bother. Just mouse over the attack and eyeball if it's a 19. As a Pro subscriber you could use the API to make a button that sets the @{d20} Attribute on the NPC sheet to 1d20cs>19 and back again?