If she gave you what appears to be a standard range, you might want to consider changing the cord instead of changing the outlet.

What is the nameplate rating on the appliance?

 

Replace the receptacle (outlet; socket) and also replace the 50 amp breaker down in the panel with a 30 amp breaker. The stove cord and plug remain unaltered.

There could be complications. If the plug for the new stove has neutral and/or ground contacts that the original cable to the old receptacle does not provide conductors for, you would need to string a new 6* gauge 3 conductor plus ground cable down to the panel before you can complete the project.

If only a ground conductor is needed and lacking you can string just a ground conductor to the panel more or less following the route of the existing cable.

* 10 gauge will carry 30 amps but it would be nice to future proof the installation for 50 amps for the next owner of the house.

 

If the new stove cord has the same number of pins as the 50 amp receptacle change the plug on the stove. It's a lot easier putting 30 amp wires in a 50 amp plug than it is putting 50 amp wires in a 30 amp receptacle and breaker.

 

I have yet to see a free standing unit require yes than 40 amps.
I'm betting someone wire the old unit with a dryer receptacle.
Check the name plate on the unit and get back to us!

 

Unless it is a gas range, the minimum required ampacity will be 40A, which requires a 50A plug and receptacle. Someone apparently miswired this initially. As others have said, check the wattage, but plan on changing the cord to a 50A cord.