Isotopes are versions of an element that share a proton count but have different neutron counts. These neutrons serve to increase the strong nuclear force keeping the protons together without increasing the electromagnetic repulsion that forces them apart. This helps stabilise the atom.
Due to the fact that elements are defined by their proton count, we can not call atoms with identical proton counts but different neutron counts elements. Instead, scientists call these variants isotopes. They have almost identical chemical properties, though the nuclear and some physical ones differ.
An element can change from one isotope into another through intense neutron bombardement. Eventually, a neutron will be absorbed and (provided the nucleus does not fission) is integrated into the nucleus. This results in a more massive isotope.
Alternatively, certain decay chains can change the mass of an atom while resulting in the same element. A combination of double beta-minus decay and single alpha decay is one such chain.
Finally, different nuclear fusion chains can result in different isotopes of the same element.