replace function does not works as well

I'm writing a function that replace specific characters to other ones

public static String makeComplement(String dna) {
if(dna.contains("A")|| (dna.contains("T") ||(dna.contains("G") ||(dna.contains("C") ) )) ){


dna = dna.replace('A' , 'T');
dna = dna.replace('T' , 'A');
dna = dna.replace('G' , 'C');
dna = dna.replace('C' , 'G');

System.out.println(dna);
}

return dna;

}
    public static void main(String[] args) {
String ex ="GTACTCC";
System.out.println(ex);

makeComplement(ex);

it replaces the T in A & the C in G but keeps A & G the same