what would happen if a mutation changed UAC (third position) to UAA?

Answer 1

UAA is a stop codon, so the protein formation would stop at that point.

Answer 2

if u mean the mRNA changing to amino acids, the amino acid, Tyrosisne will stop being produced and nothing else will be produced because there is no amino acid produced by UAA. By the way, mutations only effect further generations if messed up in the gametes.