Replacing a junction box involves several steps to ensure safety and proper function. Here’s a general guide:

Turn off the power: There is usually a circuit breaker controlling the power to the junction box, turn off the power first.

Prepare Tools and Materials: Gather the necessary tools and materials, including a new junction box, wire connectors, wire nuts, a screwdriver, wire stripper, and pliers.

Remove the Old Junction Box: Carefully remove the cover of the old junction box and disconnect all wires inside. Take note of how the wires are connected, either by taking a photo or making a diagram. Unscrew the junction box from the wall or ceiling and remove it.
Install the New Junction Box: Position the new junction box in place and secure it to the wall or ceiling using screws. Make sure it’s flush and stable.

Connect the Wires: Strip the insulation from the ends of the wires in the wall to expose about ½ inch of bare wire. Connect the wires from the wall to the corresponding wires in the junction box, using wire connectors or wire nuts. Follow the notes or diagram you made earlier to ensure correct connections.

Secure the Connections: After connecting the wires, make sure they are securely fastened with the wire connectors or wire nuts. Tug gently on each wire to ensure they are firmly connected.

Tuck Wires In: Carefully tuck the wires into the junction box, making sure there is enough space and they are not pinched or strained.
Cover and Test: Put the cover on the junction box and secure it with screws. Turn the power back on at the main electrical panel and test the circuit to ensure everything is working properly.

Safety Check: Once you’ve confirmed that the circuit is working, double-check all connections to ensure they are secure and there are no exposed wires.

Clean Up: Dispose of the old junction box and any debris safely.
Remember, if you’re unsure about any part of the process, it’s best to consult a qualified electrician. Safety is paramount when working with electricity.