I assume this is for a sprint kart. In that case, I loosen all the set screws and pinch bolts. Remove the wheels/hubs. Slide the sprocket hub off since it's on the outside of the bearings if a 2 cycle. If a 4 cycle setup, begin sliding the axle out of the cassettes / bearings and hold the brake rotor within the pads so it doesn't try to go cockeyed and keep the axle from moving. Sometimes I rotate it on the axle to keep it from binding. Slide the axle out incrementally, making sure the brake hub and rotor don't fall, and sprocket hub doesn't fall. Once it's clear of the left bearing, I usually try to rotate it and slide out of the right bearing. File any burrs off, clean and lubricate with WD-40 or similar. Slide new axle in and center brake hub. Don't forget to put sprocket hub inboard if required. Measure each end stub back to bearings -- center axle in kart. Tighten bearing set screws (I loctite mine but don't gorilla tighten). Center brake rotor in pads. Align sprocket hub. Install wheel hubs. Tighten all pinch bolts. Done.