This varies from pc to pc. I have one that has a dedicated button for changing the gpu, another must be changed in Computer Management in Windows. I suggest that this user contact the support group for his PC/GPU manufacturer for details.
As to which is better, that depends on the users preferences which may favor conversion speed over video quality or other trade offs. Tests by DVDFab staff several years ago indicated the best video quality was the Software setting, followed by Intel, followed by nvidia. Best speed was usually nvidia, followed by Intel with Software last. These test were run using BDMV sources; I do not believe the Software setting is very practical for UHD unless the user is very patient or has lots of free time.
