How do you erase part of a drawing done with an Apple Pencil ?
How can I erase part of a drawing on my iPad using an Apple Pencil?
iPad Air 2, iPadOS 13
That would depend on the App you used for the drawing. Most Apps will offer an "eraser" tool you can select to erase previously drawn things.
See the eraser head tool in the toolbar in the following image for an example: