most common way is to use a temp variable as follows,
temp = x;
x = y;
y = temp;
But you can use a simple trick to swap them without a explicit temp variable.
x=20; y=10;
x=x+y; //x=30
y=x-y; //y=20;
x=x-y; //x=10;
It worked, nice trick keep in mind, interviewers like this question.
Also XOR operator can be used to Swap without a temp variable. XOR swap algorithm is as follows,
x = x XOR y y = y XOR x x = x XOR y
