Hi,

The mass number of an atom is the total number of protons and neutrons in a nucleus.

And the atomic number is number of protons found in the nucleus.

There are two types of beta decay-

(β ⁻ ) decay -a neutron is converted to a proton, and electronis created.

beta plus (β ⁺ ) decay - a proton is converted to a neutron and the process creates a positron and an electron neutrino.

Now understand what happens in beta minus decay-

Suppose carbon 14 is converted to nitrogen 14 in the beta decay. The mass number is same of both i.e 14 but the atomic number increases by one in case of nitrogen. As it is an example of beta minus decay what happens here is neutron is converted into proton so we have mass number (proton +neutron) same. But the atomic number (number of proton) get increased by one.

Hope you find it helpful.