Rounding decimals: the 5 rule
To round a decimal, look at the digit one place to the right of the place you are rounding to. If that digit is 5 or more, round up, and if it is 4 or less, keep the digit as it is.
A worked example
To round 7.365 to the nearest hundredth, notice that the hundredths digit is 6. The digit to its right is 5, so the 6 goes up to 7, and you get 7.37. To round 8.5 to a whole number, look at the 5 after the ones place. The 8 goes up to 9. To round 2.43 to the nearest tenth, check the next digit, which is 3, so the 4 stays the same and you get 2.4. Recap: find the place, check only the next digit, and drop every digit after the place.
The mistake people actually make
Never round twice in a row. 2.449 to the nearest tenth is 2.4, because the next digit is 4, but rounding first to 2.45 and then to 2.5 gives the wrong answer. Also, 3.96 to the nearest tenth is 4.0, and you keep the zero to show the tenths place.