How do I reset velocity settings on Komplete Kontrol Mk 2?

One key is playing a lot louder than the others

Try deleting the Midi template : Managing KOMPLETE KONTROL MK2 MIDI Templates

Also if this is in a DAW then try outside the DAW e.g. in Komplete Kontrol standalone or Kontakt standalone. And if that changes anything then try reset the DAW midi settings somehow.

I am not a keyboard expert fut if that does not work then it is most likely a hardware problem.

Else then you can try contacting N.I. Komplete Kontrol Support or N.I. Hardware Support

In case that you need guidance contacting support : How to contact NI Support and How to get in touch with our Customer Care