How can a substitution mutation change a protein?

by altering codon to stop codon or changing the codon for one amino acid into codon for another amino acid

Explanation:

A point mutation is a mutation within DNA sequence when a single nucleotide is changed (substitued, deleted, added). They can have various effects depending on the type of point mutation. For example, nonsense mutations are point mutations that result in a premature stop codon. UAC codon codes for the tyrosine amino acid, while UAG is stop codon. As a consequence, of mutation (UAC to UAG) protein product is usually unfunctional because it is incomplete.

Missense mutation is a type of mutation that changes codon into codon that codes for another amino acid. This type of mutation can result in nonfunctional protein.