That really depends on the problem you are looking at. The Richardson number gives the ratio of buoyancy term and shear term. So in order to adjust your Richardson number, you could change the temperature or the velocity. The problem might be, that you would also change your Reynolds number in both accounts. But in Fluent, you could probably also change the gravity constant.
If you are looking at a flow problem on earth, changing the gravitational constant is not physically sound. But what I wanted to say is that in CFD, you can change any factor of the Richardson number. But you must take care about changes of other dimensionless numbers, such as the Reynolds number. Otherwise, you solution may change and you don't see the governing parameter behind it.