The button pops out of the knob. Get a strong thumbnail in the joint between the knob and the button's fascia and ease it out. The knob itself will come off the metal lever if you work it gently; the lever is splined.
Mine stopped working well some years ago; I'd have to press and hold the button for it to work. Upon disassembly, I found that a wire's insulation had cracked or abraded through. I replaced the wire and it worked quite well for a few years.
It's not working well again, but this time I might replace the whole lever: years ago also, the knob had cracked at the joint with the metal tube. Judicious application of foil from a cigarette wrapper tightened it up nicely, but it's worn out again. Also, the lever has been sloppy/loose for a very long time. I took it off the column once, but didn't find anything obvious. I live with it, but it'd be nice to have a taut shifter again.
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