Look on the shank of the inside knob, to see if there is a small slot, near the face plate. If so, insert a tiny screwdriver, the end of a nail file, or similar shape in the slot, and depress the spring as you slide the knob off. After that, pop the face plate off. If that doen't fly, post a brand or picture.
Look for a release on the knob, a small slot that you can push a small screwdriver into. There should be some way to remove the trim ring that is against the door. Either a small slot or dimple around the ring. Put something small, screwdriver or putty knife, in that area a gently pry out. Then you should see some screws. Good luck