Rounding decimals by place value
Rounding a decimal starts with naming which digit sits in the place you are rounding to, then looking only at the digit one place further right.
A worked example
To round 4.278 to the nearest hundredth, the hundredths digit is 7. The digit right after it, 8, is 5 or more, so the 7 rounds up to 8, giving 4.28. Every other digit is dropped, not adjusted.
The mistake people actually make
Rounding down does not mean deleting the digit; 4.278 rounded to the nearest tenth is 4.3, not 4.2, because the digit after the tenths place is 7, which rounds up. Naming the wrong place gives a wrong answer even with correct rounding rules otherwise.