Path Painter was created as a paint tool. This means that mixing paths, or painting several paths over one another to achieve different results was preferred above long term editability (the two are mutually exclusive in a way as explained later). Sort of how painters create: they paint on top of earlier brush strokes to change or even delete them (or get something new by mixing different brush strokes/colours).
The possibility to adjust directly after painting was added thinking it was a nice feature to be able to fine tune a brush stroke before moving onto the next one, but you are not able to adjust it after that, since it becomes part of "the canvas", so the next path can paint over it.
You can think of it as Path Painter is more of an artistic tool, that allows blending different things, than an engineering tool, that allows long term tweaking and tuning.
Long term editability or non-destructive workflows are, by nature, inherent of spline-based terrain tools. For example, spline based road creators. A non-destructive workflow very quickly becomes a huge mess for terrain painting. When painting, the previous height of the terrain and different settings determine the height of the path at any given point. It very quickly becomes hugely confusing when you start editing a path that's supposed to change the input of others that also affect others, in a chain reaction like fashion.
This doesn't mean that we never thought of adding long term editability for paths. In fact that has been on our minds for a long time. The above rather means it's not as straight forward in Path Painter's case. There's a good chance a non-destructive workflow would ruin most users' experience, unless some clever approaches are identified that mitigate the previously mentioned pitfalls. There are plans to try and explore a few things to achieve a best of both world or the ability to choose one direction, but it's uncertain when we will be able to do that.
Having both available would probably end up with a lot of complaints about bad usability, because it would quickly become very confusing when trying to edit paths in a complex blend painted system, like for example these river/lake beds
[TL;DR: Other than painting over them, you can't, although there's been and there is constantly a lot of thought put into making this happen sometime in the future.]