>a general solution is impossible without FreeCAD reading your mind.
The goal of a well-written program is to give the user the opportunity to express what is in their mind at an appropriate time, and to change their mind after seeing some output. And to be told explicitly or implicitly why their inputs don't work.
FreeCAD tends to get wedged so you can't back up, very unpredictably (for a newbie). This makes all its other issues worse because it inhibits experimentation.
You actually can back up, but it is time-consuming and not easy to discover. You need to manually step through the project tree until you find the first feature that is incorrect (this is not necessarily the first feature that fails to compute!), then either fix the sketch itself, or fix the sketch attachment, or the fillet/chamfer edges in the case of fillets/chamfers, and then proceed down the tree until it's all correct.
It's a lot of work, but often less work than starting from scratch.
The goal of a well-written program is to give the user the opportunity to express what is in their mind at an appropriate time, and to change their mind after seeing some output. And to be told explicitly or implicitly why their inputs don't work.
FreeCAD tends to get wedged so you can't back up, very unpredictably (for a newbie). This makes all its other issues worse because it inhibits experimentation.