# Pastebin tCZ6wfiq
parent:
  a: x
  b:
    c: y
    d: z

what you want:
  a: x
  b:
    f: q


You can use

  layeringDefinition:
    layer: site
    parentSelector:
      foo: bar
    actions:
      - method: merge
        path: .
      - method: replace
        path: .b