Hello saldavonschwartz,
That depends on whether your FirstPersonProject is created for C++ or Blueprints. If it’s for blueprints you’ll see the OnHit functionality in the Event Graph of the blueprint and the variable you need to edit should be there. If it’s for C++, you’ll find the following line in FirstPersonProjectile.cpp:
OtherComp->AddImpulseAtLocation(GetVelocity() * 100.0f, GetActorLocation());
You can edit the 100.0f here to change the velocity multiplier that I mentioned.