If the betta is already in a tank, all you need to do is move everything over to the new tank - substrate, decor and most importantly the filter. And move the betta at the same time. These contain almost all the bacteria in the current tank, you'll just lose the bacteria growing on the tank itself. Provided the tank has had regular water changes, the tank water will be pretty similar to your tap water so you don't need to move any water. But if water changes have been small and infrequent, then yes, move some water as well.
Monitor ammonia and nitrite for several days to make sure, and do a water change if either show above zero.
Another thing you could do is to get some live floating plants - salvinia, frogbit, whatever is available where you live. Floating plants do two things - they take up ammonia and don't turn it into nitrite; and they provide cover for the betta and roots for him to swim though.