If UAG was mutated to UCG, then the new amino acid would be Serine. You can find this by looking up an amino acid translation table, unless you have memorized all of the amino acids there is not another way to do this. UAG to UCG is most likely a substitution mutation because A was changed to a C. Since UAG is a stop codon this mutation will cause mRNA to translate too many codons, resulting in a protein that is too long.
