First locate the silver ring that connects the top of the shift boot to the botom of the stock knob.
The ring is secured to the knob with two clips. Gently pull the ring down (away from the knob) to unclip it. Once the ring is unclipped, simply unscrew the knob by turning it counter clockwise (lefty loosey).
One thing to note when installing an aftermarket shift knob. There is no longer anything securing the top of the boot (unless you by a knob that clips into the ring), so the boot will sag a little.
I hope this is helpful.