Should I replace my 30 AMP with a 40 amp circuit breaker for a new glass radiant heat cooktop? Now I have 30 amp just for the glass cooktop. Best Buy recommended I use a 40 amp after a pre-installation check-up.
You don't ever change the breakers to match the appliance !!
You can only change the breakers after you upgraded the wires to carry the needed load.
I have an induction cooktop, and I needed to upgrade the wires to a bigger gauge. Luckily, it was just 6 feet. Costly, but that's the only way.
If you change the breakers but not the wires, your wires will overheat and your house burn down. The insurance company will find this post :-).
More seriously, upgrade the wires first(1), then up the breakers, then enjoy your cooking.
Edit: (1) More precisely, make sure they are the proper gauge. There's some subtlety there. The run-of-the-mill 8-gauge solid copper as sold in my area is good for 40amps. But I have no clue what type of wires you have in your area.
If the Installation Instructions say to use a 40A circuit then you should feed it with a 40A breaker and appropriate wire.
The NRTL (UL,ETL. or other) Listing is only valid when used as tested per documents that are included with the product, those instructions are part of the Listing, and electrical codes require adhering to those instructions. You can download those instructions for most all appliances. Many products directly specify minimum breaker and wire type, but some cooktops can be a bit complicated. There are odd provisions in the NEC that allow some cooktop circuits to be provisioned to share a circuit with some other cooking appliances. So the instructions may just specify minimum circuit ampacity (capacity) required for the appliance and include a statement saying to consult national and local electrical codes.
If the instructions do specify a breaker size (without specifying wire size) then the minimum wire size required is based on the insulation rating of wire, type of wire, and temperature rating of receptacles if used.
So more details are needed for a definitive answer.
Check the wire size
Typically one finds 10 AWG wire to a cooktop. That is not big enough for 40A and the maximum breaker it can take is 30A. (NEC 240.4(D)).
However if the wire is 8 AWG copper or 6 AWG aluminum, then a 40A breaker is allowed. We tend to view an appliance installer's advice as suspect, but the appliance installer actually had eyeballs on it.
In rare cases 40A may be allowable on #8 aluminum, but it would have to be an unusual wire type such as SE. Regardless, aluminum needs special aluminum-rated splices; copper-only lugs are intolerant of aluminum and will start a fire.
And check the instructions
As NoSparksPlease says, the instructions MUST be followed. They will typically specify breaker size and wire feed size.
If they don't state wire size, that is NOT a blessing to use existing wire beyond its rating.