Is it easy to change rear wheel bearing Hub or should I use mechanic

I presume its a drum brake? if so, it could be a faff, but could be done?

Disc

In which case, the hardest part is going to be winding back the pistons without damaging the sleeves.

If you have the time, you can do it, but it is probably going to take you a couple of hours.

All Mondeo mk3s are discs all round, the disc sizes are the same from the TDDI to the ST-220,

Its easy if you have the right tools, skills etc, a competent DIY mechanic could tackle it ok, ruster/ siezed nuts/ bolts can be a problem,

if you are not changing the discs/ pads the pistons might not be too much of a problem

A Haynes manual is handy for jobs like this

You can wind the pistons back in with an angle grinder spanner. There are two ways I would suggest doing it. First method is to undo the bleed valve as you wind the piston in, with a length of tube to direct the fluid into a container - you don't need to worry about flipping the seals in the master cylinder that way. The second method is to remove the flexi hose from the caliper (clamp the hose off first) as it's much easier to wind the piston in by standing the caliper on its end so you can push down as you turn the spanner.

Of course, if you aren't changing any of the brakes and you're ultra-careful to not touch the brake pedal while the caliper is off the car, there's no reason to think you won't be able to just bung it straight back on afterwards.

Back to the original question, though. The hub is held on with four Torx-head bolts, and from memory, access is through a hole in the front plate of the assembly so you'll need some of the longer torx bits and/or a long thin bar.