Can I use a 100AH battery instead of a 74AH battery in my Passat 2.0TDI, 2012

Yes, you can use a 100AH battery in place of the 74AH battery as long as it fits in the physical location and the post location is in the same orientation. Most batteries which have more energy store are physically bigger than their weaker brother. This is because to store more energy, you have to have more space (in most cases). Double check to ensure there's no issues there and you should be golden. Having more energy reserve will cause absolutely no issues with vehicle or the electronics of the vehicle.