The electrician would just run new cable (wire) from the panel to the new cooktop.
The new cable would need to be of the proper gauge for the load, and the breaker would need to be upgraded.
For 40 amp, the gauge would be #8 and for 50 amp it would be #6.
The old cable would be removed, if possible, allowing the the new cable to be run in it's place, if possible. If the existing cable is run in conduit, there are some restrictions as to whether or not the size of the cable allowed in conduit. But, your electrician will know all that.