On PC, go to your user folder, then go to Appdata (may need to view hidden folders to see this) > Local > Sony Corporation > Ps4 remote play. Delete the settings and data files, and you'll be able to change user.
I don't have a Mac, but if you have a Mac, then I think you have to go to the application data folder, then find the "Sony Corporation" folder (or just search for it), then go to the Ps4 remote play folder, delete the settings files and then it will work.