There are a variety of axle types. The basic two are finished on the ends by a crimp, or are turned over into a dome, looks like a rivet head. There are some that fit snugly into the body shell, those that make changing tires impossible. The axles also fit differently into the chassis. I shall try to list them all.
These are fitted as part of the chassis, removing the chassis and the axles are attached.
These are held into the body and have to be removed before the chassis can be detached.
These are held into slots in the body shell, removing the chassis and the axles come out separately.
The crimped axles can be replaced using a vice grip that has flats on the jaws. See the video, it requires a strong hand but is effective. The axle ends must be filed flat and smooth to get the best result. One end can be crimped before insering into the model.
Here is the tricky one but with practice a good result can be achieved. A ball pein hammer, good quality, has to be used. The axle ends can be rounded by tapping the edges with the ball on the edges of the axle end and in a circular motion around the end. A gentle tap occasionaly directly on the end will help to spread it. One end is easy, the other has to be done with the axle in place.