Select the "Crit Damage Option" extension when loading your campaign, then select the option you want (it's off by default).

Currently supported options:

• Perkins: the extra crit damage will be set to the maximum (e.g. if you do 2d6 damage, you'll get whatever you roll + 12)
• Exploding: for every damage die rolled, if you roll the max, then you get another (die of the same type)
• Cascading: similar to exploding, but the new die will be one smaller (e.g. if you roll a 6 on a d6, you'll get a bonus d4)
• Inspirational: a random ally in the Combat Tracker gains inspiration (damage is not affected)