Hi!
Okay, so our thousandths place is going to be the third number behind the decimal point (the first six).
Now, to round, we look at the number next to the place we're rounding up to. If that number is 5 or more, we'll add one to the number we're rounding up to and then terminate everything after that.
If the number is below 5, we don't change anything. So, look at the number next to the first six. Is it 5, or more? Yes!
This means we can change the six to a 7, and then terminate everything after it.
0.416666667 rounded to the nearest thousandth equals 0.417.
Hopefully, this helps! =)