For an isotope of a certain element, only the number of neutrons are changed, the number of protons stays the same because if you change the proton it would mean that it is a different element already. The number of neutrons is calculated from the number shown in the isotopes. That number represents the total protons and neutrons.
Iodine-127 : protons = 53, neutrons = 74
Iodine-126 : protons = 53, neutrons = 73
Iodine-128 : protons = 53, neutrons = 75
