I want to put a new gear knob on however whenever I twist the old one it does not detach and takes the whole of the underneath part with it. How do I take it off?

The gaitor must be attached to the knob with a collar.
You'll need to unclip the gaitor base from the console first.

Sent from my SM-G965F (S9+)

51 minutes ago, Stoney871 said:

The gaitor must be attached to the knob with a collar.
You'll need to unclip the gaitor base from the console first.

Sent from my SM-G965F (S9+)
 

Thanks but I have no idea how to do that is there anyway you could help me ? Step by step or something like that ? 

Unfortunately I'm not a Fiesta driver so can't be specific.
In genersl the material gaitor is attached to the knob using a clipped on pladtic ring that is nigh on impossible to remove without bresking something., (Earlier versions just used an elasticated collar that hooked into a groove at the bottom of the knob).
The genersl way to seperate the knob on later models is to unclip the base of the gaitor from the centre console and then the knob can unscrew and the knob, gaitor and base come off as a complete unit.
Fitting an aftermarket knob will mean either modifying the gaitor to attach to the new knob or sourcing an aftermarket gaitor that will match up with the new knob.

Sent from my SM-G965F (S9+)