Having the model & it's feature tree would be helpful. A sketch showing the changes you intend would also help.
Modifying the shape of the existing model is probably going to be harder. You could possibly use the Replace Face command, but I suspect that you are going to end up building an almost completely new body. You can use the Insert Part command to bring a new body into your part. You can then remove the parts of the old model that you don't need and merge the two bodies.
Hello, I obtained a model car made of two parts. The upper and lower parts are connected by screws. There are four holes on the upper cover of the car. The screws go through the holes to connect the lower half. In case I want to change the shape of the upper part of the card, what is the best way to do so? There are two scenarios. The first one is to somehow modify the shape of the existing upper case. The second one is to merge another model on the existing upper case. That way, I keep the difficult-to-make shape from the new model while using the existing holes from the original upper case. Any good sources on how to do the two scenarios? Thank you.