Existing 40 amp breaker with a 50 amp range spec?
In general, no. Installing a 50A breaker on wiring only designed to support 40A is dangerous and can result in a house fire. 40A circuits need #8 copper wire, whereas 50A circuits need #6 copper.

It's possible, although very unlikely, that the wiring used in the circuit is #6 and thus could support 50A (maybe somebody was planning for the future), but for some reason a 40A breaker was used. The only way to know for sure is to open it up and look at the wiring. The size and other specs should be printed on the sheath; as isherwood mentioned, you're looking for AWG #8 or something similar.

In the more likely case that the existing wiring is #8, you'll either need to pull new #6 wire or get a different (lower-powered) stove that will work with the existing 40A receptacle.