Feeling really silly - Cannot resize part of a part

I am totally new to Onshape. It seems really great. I imported a part, then got it mostly modified. I just need to resize the "D" hole in the middle to be smaller. I am not a CAD person, so you can imagine how hard this is. The link to the part is https://cad.onshape.com/documents/96050bba0e23a217ea239cc6/w/5c479ca8151fbb626ee87bde/e/044d39529ed66ca03a122c27

I am simply looking to shrink just the D hole in the middle of the gear without changing any other element of the parts scale. I have tired

Basically, I am trying to reduce the hole in the middle of my gear by 25% while maintaining the D shape and the size of the rest of the components/elements (sorry if this is the wrong term)

Any help would be greatly appreciated.