Here are the basics.
1.) Look at the last digit in the number. Example: 23.421 (1 is last digit). If the number is 0, 1, 2, 3, or 4, you don't do anything and just drop that digit. Example becomes 23.42
2.) The next rule is more complicated and there is an easy way and more difficult, but accurate method. I'll start with the easy way.
If the last digit ends in 5, 6, 7, 8 or 9, just round up to the next highest number by adding 1 to the number before the one you are dropping, 23.655 becomes 23.66, or 46.99 becomes 47.0, 65.6 becomes 65.7
More accurate way:
When the first digit dropped is 5 and there are no digits following or the digits following are zeros, make the preceding digit even.
42.415 rounds to 42.42 (1 becomes even number)
42.485 rounds to 42.48 (no change since 8 is already even)
In most cases, the simple way is good enough, but to be more accurate use the odd/even rules with things ending in 5.