The number of protons would increase by 1 and the number of neutrons decrease by 1.
The only time an electron would be in the nucleus is if a Neutron changed into a proton. It would then release an electron in the process. because of this, the number of neutrons decreases by 1 and the number of protons increases by 1.
Neutron -> Proton + Electron
The other answer is wrong because it doesn't take into account the part about the nucleus of the atom. If the electron was just in the normal spot, then the number wouldn't change. However, since you specified that it was in the nucleus and the only way it would be there is because of radiation. So, now that that's established, let's take a look at an example.
For example, and this wouldn't happen in Carbon 12, but let's say that Carbon 12 was radioactive and decayed using Beta Decay.
6 protons, 6 neutrons, 6 electrons (the structure of Carbon 12)
Now let's say that 1 neutron decays from a neutron to a proton and releases an electron. The new structure would be
7 protons, 5 neutrons, 6 electrons +1 electron
The reason I wrote the extra electron as +1 is because it would go flying out of the nucleus too fast to bond.
Tl;Dr: Because of Beta Decay, one neutron changes into a proton and releases an electron.