What changes take place in an atom during a fusion reaction?
There are several different fusion processes each of which generates heavier elements.
An atom is actually an atomic nucleus will a number of bound electrons. The temperatures and pressures required to sustain nuclear fusion are far to high for any electrons to stay bound.
The commonest form of fusion is stars such as our sun is proton-proton fusion. Under the high temperatures and pressures found at the core of the sun, two protons can get close enough that the electromagnetically repulsion between the two positively charged protons can be overcome by the strong nuclear force to form a Helium-2 nucleus.
Helium-2 is highly unstable and most nuclei decay back into two protons. Occasionally the weak nuclear force is able to convert a proton into a neutron, a positron and an anti-neutrino. This results in a stable Hydrogen-2 or Deuterium nucleus.
Deuterium then fuses with other protons or other deuterium nuclei to form Helium -4, the main reaction product.
For the Helium to form an atom it needs to escape the Sun and cool down to the point where it can capture two electrons.