The Kobra 3 uses a quick-release nozzle on a direct drive hotend rated to 300°C. It ships with a standard 0.4 mm brass nozzle and also supports 0.2, 0.6 and 0.8 mm sizes for finer detail or thicker, faster extrusion. Buy a nozzle made for this quick-release fitting, not a generic threaded nozzle that happens to be the right diameter.
Unload the filament and let the hotend cool fully before touching it. A quick-release design still means handling a component that was recently hot, and brass threads cross easily when everything around them is still warm and soft.
Follow Anycubic’s own Kobra 3 hotend or nozzle guide on its wiki for the exact release sequence. In general that means powering down, accessing the toolhead, releasing the old nozzle through its quick-release mechanism, and seating the replacement squarely so it sits flush against the heater block with no gap.
After installation, set the new diameter in two places: on the printer’s own screen and in the Anycubic Slicer Next profile, or whichever slicer is in use. A 0.6 mm nozzle running against a 0.4 mm profile under-extrudes or over-extrudes depending on which way the mismatch runs, and the result looks like a calibration problem when it is really a setup problem left over from the swap.
Run the resonance and flow calibration again, then print something small before trusting the new nozzle with a long job. Stop if plastic leaks above the threads, if temperature will not hold steady, or if the quick-release does not lock down firmly. Those are installation faults, not settings to nudge.
Choosing a replacement size is its own decision. The general guide to nozzle sizes beyond the stock 0.4 mm covers what changes in practice, and the comparison of brass versus hardened steel nozzles is worth reading before choosing a wear-resistant option for abrasive filament. Once the new nozzle is calibrated and printing cleanly, fold the swap into the regular maintenance schedule so wear gets caught earlier next time.