looksliketired

Hello, unfortunately there is no way to edit the path yet. You simply have to redo the entire shape or manually adjust the shape by selecting each node and moving it around.
For me, this was the first thing I noticed that was missing from AD since I'm illustrator and I rely on editing paths. Without editing the path, pencil tool is almost useless. Here, I started my own topic and request about the matter:
 
 

I will keep this in mind thanks a lot. it's just a couple of days I started to use affinity apps. watched lots of tutorials, but still trying to adapt. thank you again.

Welcome to the Serif Affinity Forums, @baris.sehri. 
 
As noted on the previous page of this thread, these brushes are vector brushes. You can't use them at all in Affinity Photo, and you need to be in the Draw Persona to see and use them in Affinity Designer.