You look at the next digit to the right. If that digit is less than 5, then the digit in the indicated place value doesn't change. If the next digit to the right is greater than or equal to 5, then the digit in the indicated place value goes up by 1. Next, all of the digits to the right of the indicated place value are removed.
Example:
123.45678 rounded to the thousandths place
6 is in the thousandths place
The next digit over (7) is greater than or equal to 5, so the 6 rounds up. Everything to the right of the 6 (now rounded up to 7) disappears, leaving:
123.457