It’ll take about £100 worth of specialist tools and parts to service.
You need to take it to bits (with the special tools), then take out the races, then measure the thickness of them, then order the races, then put it all back together again. Suffice to say, paying someone £160 or whatever to do it seems easier.
You should be able to tap out the lower bearing with a metal rod and mallet. Take it slow.
Get the genuine Canonndale headset bearings, the other ones are slightly larger and a pain to install / installing will wreck them.
You’ll need a pipe to tap on the bearings properly without messing them up – aluminium scaffold pole does the trick (make sure it is the right i.d. of course).