that was a very open-ended post. People replace dropouts all the time, it's a fairly involved process requiring a lot of care. Dropouts are all different, it generally doesn't work to just replace one set with another because the angle between the seat stays and chain stays are different between the different dropouts. Cantilever posts are not something I would recommend for a beginner, but they are fairly straightforward. There is a thread on here about canti posts that you should be able to find that talks about the low-tech ways of installing them.