How could an up quark turn into a down quark through practical means? When I say practical means I mean not waiting for some natural occurrence like in a star or something like that, but rather something like a particle accelerator.
How could an up quark turn into a down quark through practical means?
It cannot. Quantum mechanics is not deterministic, it depends on the probability of the decay, in this case, happening.
Look at the decay
It is a weak decay, and in addition it goes to a virtual W, and the probability of this happening is very small due to the weak vertex and the mass of the W in the propagator. This also means for the decay to happen there should be available energy in order to cover the mass difference, then one can have an up going to a down quark as in:
The stability of protons depends on quantum number conservation laws as the baryon number. See this for protons.
In addition, QCD, the strong force that rules at the quark level, does not allow free quarks, so a quark cannot be free, even using the probability functions.
One can make electron neutrino beams, using decays of other particles ( according to the probability of the interaction) but as quarks cannot be free, one cannot have quark beams in any sense.