DNA carries the instructions for how and when to make the principal components of cells, called proteins. Different organisms differ in their DNA instructions (composed of DNA ‘letters’, technically called ‘base pairs’) such that they make at least some different proteins.
To change an organism into a different kind, you would have to have a mechanism for changing the letters. For evolutionists, the ‘only game in town’ to change the letters is mutation. Mutations are accidental changes to the instructions, which can be one letter at a time, or multiple letters at once. Letters can be swapped, deleted, or added. Obviously, to change an organism into something more complex, letters would have to be added, not just swapped or deleted.