beta decay by emitting an electron is called as β⁻ decay. When this happens, a neutron of the element converts into a proton by emitting an electron. Hence, the mass of daughter nucleus is same as parent atom but atomic number/number of protons is higher by 1 than atomic number of parent atom.
Ni has atomic number as 28.
After β⁻ decay, the daughter nucleus will have atomic number as 28 + 1 = 29 which belongs to copper.
Decay formula is,
₂₈⁶³Ni → ₂₉⁶³Cu+ ₋₁⁰β