"It's not as simple as just upgrading the circuit breaker, and I assume you realize that. I just want to make sure other readers don't get the wrong idea. In addition to the new circuit breaker, the wiring has to be sized properly also. You'll need 8 gauge wiring for a continious 40 amp draw. A circuit that was wired for a 30 amp draw may have 10 gauge wiring, which is insufficient if you upgrade to a 40 amp EVSE."
I used 8g THHN wire for the 14-50 plug. It's good to 50amp, which when derated by 25%, means it will support a 40amp charger.
See this chart:
I'm not an electrician, but I've always gone with the following order of presidence: equipment, plug, wire, breaker. First determine the equipment you want to power, then figure out which plug you'll need (there are a dozen different plugs available with different power/voltage ratings), then figure out the gauge of wire needed for the plug, and finally which circuit breaker you need to protect the wire. I started with the Juicebox 40 which needed 50amps.