I have rebuild the spindle on my Rockwell, it is similar but may not be the same.
You will want to at least loosen and remove the retaining nut (131) while still on the mill with the quill locked. It probably takes a spanner, and could be very tight. I ended up making a custom spanner for mine.
After that you drop the quill, which can be done after the feed shaft (113) and related parts are removed. It will simply drop out. It has some weight so be careful.
There is likely a circlip near the top of the spindle that has to be removed, above the upper bearing, then it should drop out (or can be pressed out) of the quill.
There is generally a nut securing the bottom bearings on the quill. Once removed you should be able to pull the bearings.