Edit/modify existing CAD design

I am looking for help modifying two existing CAD designs (.step and SLDPRT).

https://grabcad.com/library/turbina-pelton-3
https://grabcad.com/library/pelton-turbine-6

I plan to use these designs to be 3D printed, so they need to be modified to facilitate that use case.

The modifications include:
1) Add/modify center hole for the shaft to be 25.5mm.
2) The diameter of the entire turbine should be 304.8mm.