Usually its the Tower Supervisor who makes the decision based on the actual and forecast wind. For noise abatement reasons, some airfields have preferential runway directions so they will continue to use the runway under certain conditions even if the wind has "gone round".
What actually happens varies depending on the airfield. At a small airfield outside controlled airspace it may just mean ATC telephoning a few agencies, such as the AFS, and sdvising them of a runway change. Signals square (if the airfield has one) would be changed and airfield lights switched (a 2 minute job). At a major airport the change could take half an hour witj many agencies needing to be advised and liaised with. Major routes around the terminal area need to be changed, etc, etc. However, it usually runs pretty smoothly!!