If you have a new theme that you want everybody to see, say for example it is Christmas and you have a nice Christmas theme, the easy way to set all users to the new theme is to set that theme to default theme in admin panel -> Admin Assistant settings and then go to General Settings and change the theme cookie name. That will cause all users to lose their theme cookie settings so will automatically load the new default theme in its place.

This won't affect the user's theme setting in the profile however. To change that, you would need to uncheck the "Allow User to Change Theme" setting. When that is set to off, the default theme is loaded.