That should be relatively easy to replace. There is a cap on the end of the Mitre Adjustment Knob, so you have to remove that first.

Once removed you should see a screw inside of the knob that holds it on. Just remove this screw and you should be able to remove the old Knob and install the new Knob.

Let me know if you need any additional information and I'll do my best to assist further.

Attached is a breakdown of this saw for your reference.

- MTR