The opencpn.ini file in a Windows computer is usually located:
By default, Windows hides the "C:\ProgramData\" folder. I think you have found the location of "opencpn.ini" on your computer. If not, the using the help (?) button on the toolbar as suggested by bcn will help you find it.
Your (edited) message indicates that OpenCPN changes the
setting back to the original setting. This would be the case if OpenCPN is running when you edit the opencpn.ini file. OpenCPN reads opencpn.ini into memory when it is started, and it writes it from memory as it shuts down. So, OpenCPN must not be running when you edit and save opencpn.ini.