Lap changing is fine, if you are secure that baby won't roll off.
If you prefer a firm surface, kneel down with your left side against an armchair settee.
Kneel by baby's side so she has her head towards the settee, if you are right handed - reverse if left handed - and very gently place your right knee across her torso, with your body weight on your left knee which will keep her on her back, and leave your hands free.
Lift her legs and hold them together in your left hand at the ankes, and you have the other hand free to wipe, cream, and place new nappy.
Make sure you are properly balanced and there is no chance you will roll over onto her. This way she can;t roll one way, the settee or chair is in the way, and if you keep her feet raised, she can't get purchase to roll the other way.
Change as fast as you can, but stay calm, if you get in a state, so will she. Try and talk to her quietly and praise he if she lies still - she ill associate being wstill with your praise, and that should be the worst part over.