Whilst I do not know I suspect minimal. It's just a case of removing the old valve and screwing on the new. I can't recall if one needs to do anything special to stop the water flow but the first turn or two of the nut should indicate :-) As far as I can recall I've only ever needed to change the knob (or else I'm taking rads off and drain the system anyway).
What's wrong with the TRV? If it's not working it may be stuck. Unscrew the top of the TRV and there should be a small spindle sticking up in the centre. These spindles have a habit of sticking down, particularly if the valve has been turned off for some time. Press the spindle with some tool to see if it moves up and down freely. If it doesn't a few taps may free it, or a squirt of WD40.