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The grease in old cameras often hardens with age, making the focus helical stiff or even frozen.
Around the ring, you will see three tiny holes, and a grub screw is hiding in each one. Loosen those, and the focus ring will come off, revealing the focus helical. Then you need to somehow unscrew the helical and clean the threads with a solvent (I use naphtha). But carefully mark the positions of everything as you remove parts so that the focus-calibration will still be correct after re-assembly.
Mark Overton