How do I replace the axles?

One kind: Remove axle pin (under hubcap) & slide out.

Another : The axle has a flat-indentation that will not allow removal from the braces. You must remove the braces & turn them to align the slots.

Yet another: more modern: A spring-steel clip that 'bites' the axle. Often has a plastic or tin cover. Replacements found on eBay.

Post a picture... get an exact answer. The years of Radio Flyer is more than me !