I am getting ready to have a new double wall oven installed. The current one uses a 35 amp circuit breaker. The new one requires a 40 amp breaker. The electrical box is Square D QO. The new breaker will be a double pole. My question: Can I just replace the breaker or does the all the wiring need to be changed also? Thanks for your help

It really depends on the AWG of the existing cable. A 10 AWG cable is listed for up to 30 amps. An 8 AWG cable is listed for 40 amps.

Also, if the existing cable is 3 wire may cause you to have to replace it any way, if the new appliance require a four wire cable and receptacle.

Agree and the chances are slim to none that you have 8 awg. Who is doing the install?