Very easy.
Bought the wheels, about 10" in diameter.
The inner hubs were slightly smaller than the standard axle on the Cimex, so I took them to an engineering shop where they reamed them out to size.
The silver hub caps can be prised off with two large screwdrivers, coming down from the top, one on each side of the axle.
(The hubcaps have a tension spring inside of them and are easily put back on when the new wheel is on.)
Once the caps are of, the wheels are pulled off and the new ones put on, then the hubcaps replaced.
I do not run my tyres with full pressure, but about 2/3, this allows the wheels to flex over the step edge.
Also, being larger, they don't get caught up under any steps that may have bullnose edging.
I'll try and post some pics for you.
My wheels are made by Fallshaw.
Try to source them from material handling stores, disabled stores such as those that sell wheel chairs, also some sports stores with pull along golf buggies.
Some people comment that the big wheels puts the Cimex off balance, this is not true as the Cimex has a floating head that compensates for all wheel heights within reason.
Hope this helps.
Ooroo,
Shorty.