I have an existing cooktop with a 50 amp breaker and #6 wire Replacing cooktop with a new induction top that requires a 40 amp circuit Is it ok to leave the 50 amp breaker box or should I swap with 40 amp breaker? Thank you
Breakers often are tasked with two missions; protecting the conductors is always #1, and sometimes they are there to protect the equipment itself. Going from a 50A breaker to a 40A breaker is no problem from the standpoint of the protecting the conductors; you can always go down in breaker size, just never up.
So that leaves the protection for the cooktop itself. In the installation instructions (assuming you have them) or possibly on a nameplate on the unit, there should be something telling you Minimum Circuit Ampacity (abbreviated as MCA) which is addressing the MINIMUM wire size as discussed above, and there might be another listing of Maximum Over Current Protection (abbreviated MOCP) that is dealing with the protective device (fuse or breaker) that might be required in it's listing. So we cannot answer this for you other than to say "RTFM" (Read The Factory Manual).
You're always allowed to use larger wire than necessary. By the book, a 40A circuit calls for 8 AWG cable. You already have 6 AWG in the wall, and that's bigger, so the existing wire is totally acceptable for 40A.
Just change the breaker and recep-- oh wait. There's no such thing as a 40A receptacle. So there's an exception in Code that says "On 40A circuits, use the 50A receptacle". That was easy!
On the breaker... Consult the installation instructions. It may say "Use 40A or 50A breaker" and then you are done. If it says "Requires 40A breaker" you must change the breaker, because a 50A breaker won't protect the appliance. However it's a $10 breaker.
What's that all about? When they design the oven, they send the first run to Underwriter's Laboratories for a UL listing. UL looks at the design, gives advice, tests it in the conditions the manufacturer pays for, and certifies it as safe in those conditions. Those are the conditions described in the labeling and instructions.
Lastly on the wiring connection. Some ranges use a hot-hot-ground connection and those are fine. Most use either a hot-hot-neutral (no ground) or a hot-hot-neutral-ground connection. For those ungrounded connections, we very strongly advise converting to a grounded connection. If the ground wire (or metal conduit ground path) isn't present, then retrofit the ground wire.