Hi. It's not particularly difficult, but it does depend of the wall. If it is brick you may need to insert a lintel above the new opening. If it is structural I guess it would be a certainty. Go ask on the general building forum for advice.
If the wall is just plasterboard, then it is simply a case of removing the old door together with all the surrounding woodwork (the casing and architrave) carefully, cutting the new hole (avoiding cables pipes etc) and fitting the new casing etc. Plasterboard cuts easily with a Stanly knife.
Fit some noggins across the old hole, cut a couple of sheets of plasterboard to size and screw (using plasterboard screws) in place
You will damage the plaster, so budget for a plasterer to come in and make good.
Basically the trick is to carefully look what you are doing when dismantling the old door fitting, so you see how the new one goes together.