There is no step by step guide to this, the manufacturer assumes the engineer can figure it out.
Isolate the power to the appliance.
Take the oven out of the housing and take the top panel off.
Take the fixings holding the facia panel on out and take the facia panel off.
(you can disconnect the wires to allow you easier working on it).
The push button is only held by snap open plastic, as you are replacing it you can cut the old bits off and push the button out.
Fit the new button, refit any parts removed and connect the wiring up.
Refit the facia, refit the top and refit the oven to the housing.
Restore the power and set the clock.
It is possible, but a lot more difficult, to do this without taking the facia off.
It's not a difficult job to do, the main thing to watch is the razor sharp edges of the appliance whenever a cover is removed.
Take it apart, have a good look at it and just post here if you get stuck.
Hope this helps
Let me know how it goes.