My headset (i.e the bearings in the steering tube) needs replacing. Is this a job requiring special tools like a press etc or is it a DIY job?

The frame is a Trek Superfly FS 2014 model.

Secondly if it can be done as a DIY job, how do I know what bearing dimensions to get? In the past I tried to be clever and bought online for another bike just find out it did not fit and will try to avoid that mistake again.

Any advise most welcome.

Cheers!