In Windows, you set the R_USER profile by opening up a command line and running:
SETX R_PROFILE_USER "C:/.../.Rprofile"
Where (obviously) the path is the path to your desired .Rpofile. In R, you can check that it worked:
Sys.getenv("R_PROFILE_USER")
Should return the path you specified. Note that you likely need to have all R sessions closed before setting the R_USER variable.